Memorable quote (s) from the movie:
David Dobel (Woody Allen): Since the beginning of time people have been, you know, frightened and, and unhappy, and they're scared of death, and they're scared of getting old, and there's always been priests around, and shamans, and now shrinks, to tell 'em, "Look, I know you're frightened, but I can help you. Of course, it is going to cost you a few bucks...” But they can't help you, Falk, because life is what it is.
Monday, July 09, 2007
Brazilianite
Chemistry: Hydrous sodium aluminum phosphate.
Crystal system: Monoclinic; short prism; large spear-shaped.
Color: Transparent to translucent; colorless with striations; yellow/green, yellow, colorless (rare).
Hardness: 5.5
Cleavage: Perfect: 1 direction, parallel to pinacoid faces; Fracture: brittle, conchoidal.
Specific gravity: 2.98
Refractive index: 1.603 – 1.623; 0.02
Luster: Vitreous.
Dispersion: Low.
Dichroism: Weak (merely a change in shade).
Occurrence: Hydrothermal in pegmatite cavities. Brazil, U.S.A.
Notes
Collector's stone; heat sensitive; first found in 1944; may look like beryl, chrysoberyl, topaz, but R.I and DR different.
Crystal system: Monoclinic; short prism; large spear-shaped.
Color: Transparent to translucent; colorless with striations; yellow/green, yellow, colorless (rare).
Hardness: 5.5
Cleavage: Perfect: 1 direction, parallel to pinacoid faces; Fracture: brittle, conchoidal.
Specific gravity: 2.98
Refractive index: 1.603 – 1.623; 0.02
Luster: Vitreous.
Dispersion: Low.
Dichroism: Weak (merely a change in shade).
Occurrence: Hydrothermal in pegmatite cavities. Brazil, U.S.A.
Notes
Collector's stone; heat sensitive; first found in 1944; may look like beryl, chrysoberyl, topaz, but R.I and DR different.
Brazilianite
Chemistry: Hydrous sodium aluminum phosphate.
Crystal system: Monoclinic; short prism; large spear-shaped.
Color: Transparent to translucent; colorless with striations; yellow/green, yellow, colorless (rare).
Hardness: 5.5
Cleavage: Perfect: 1 direction, parallel to pinacoid faces; Fracture: brittle, conchoidal.
Specific gravity: 2.98
Refractive index: 1.603 – 1.623; 0.02
Luster: Vitreous.
Dispersion: Low.
Dichroism: Weak (merely a change in shade).
Occurrence: Hydrothermal in pegmatite cavities. Brazil, U.S.A.
Notes
Collector's stone; heat sensitive; first found in 1944; may look like beryl, chrysoberyl, topaz, but R.I and DR different.
Crystal system: Monoclinic; short prism; large spear-shaped.
Color: Transparent to translucent; colorless with striations; yellow/green, yellow, colorless (rare).
Hardness: 5.5
Cleavage: Perfect: 1 direction, parallel to pinacoid faces; Fracture: brittle, conchoidal.
Specific gravity: 2.98
Refractive index: 1.603 – 1.623; 0.02
Luster: Vitreous.
Dispersion: Low.
Dichroism: Weak (merely a change in shade).
Occurrence: Hydrothermal in pegmatite cavities. Brazil, U.S.A.
Notes
Collector's stone; heat sensitive; first found in 1944; may look like beryl, chrysoberyl, topaz, but R.I and DR different.
Code of Practice Favoring Cultured Ambiguity
Chaim Even-Zohar writes about The Council for Responsible Jewellery Practices (CRJP) + its Code of Practices + gaps and shortcomings + other viewpoints @ http://www.idexonline.com/portal_FullEditorial.asp?TextSearch=&KeyMatch=0&id=26245
Sapphire Mining In Chantaburi (Thailand)
2007: Chantaburi has changed a lot since 1973. There are new types of colored stones coming from Africa, South Asia, and Southeast Asia + good quality rubies and sapphires are getting more difficult to find + foreign tourists, dealers, students are still visiting Chantaburi looking for the best deal and making mistakes.
(via The Journal of Gemmology, Vol.13, No.8, October 1973) J A L Pavitt writes:
Thailand, or Siam as it was formerly named, is a well-known source of sapphire, ruby, star-sapphire and zircon and over the years the skill of Thai lapidaries has advanced to a very high degree, making Bangkok, the capital city, an important center for the supply of cut gemstones.
There are a number of gem mining localities in Thailand, many of them in remote areas, but the mines at Chantaburi (also known as Chantabun), 200 miles from Bangkok, can be reached by car in five and a half hours, and soon after our arrival in Thailand in 1971 my wife and I made our first visit to Khau Ploi Waen, or ‘Hill of the Sapphire Ring’, as this mining area at Chantaburi is named. I have since made further visits, the most recent in January 1973 with Mr Kenneth Parkinson during his two week visit to Thailand.
Chantaburi has a very special place in the history and culture of the Kingdom of Thailand. Situated near the coast, only thirty miles from the border with Cambodia, its inhabitants, although loyal and proud Thai nationals have ethnic origins connecting many of them with the diverse civilizations which existed thousands of years ago between the borders of China and the Mekong Delta. These origins are still evident in the customs, skills, religious and dialects to be found among the people of this fertile eastern region of the Kingdom.
It was at Chantaburi that King Taksin marshaled his forces after the fall of the ancient city of Ayuthaya, and finally defeated and drove out the Burmese invaders. Close to the sapphire mines one can see the rusting cannon and remains of the fortress of King Rama III (1787 – 1851).
The gem-bearing deposits at Khau Ploi Waen are about six miles south of the town of Chantaburi, near the village of Ban Kacha. Past records indicate that in 1850 the Shans and Burmese were extracting sapphires here and that in 1850 a British Company obtained a lease but failed to make a success of the venture. In 1919 the Siam Mining Act came into force and since then mining has been solely in the hands of Thai nationals.
The mining area is privately owned and has been cleared of primary growth and planted with rubber trees, although it is obvious that an income from rubber tapping is of minor importance. A lease to dig for gemstones over an area of one ‘rai’ (approx. 0.4 acre) for one year is granted by the landlords for a fee which may be as high as baht 300000 for high yield areas which have not previously been worked. A lease is usually shared by groups or families and there are said to be some 2000 people mining around Khau Ploi Waen.
The method of extracting the stones is very primitive, as are the tools—a pick, a spade and a rattan basket. A vertical shaft of about four feet in diameter is dug in the red/brown clay soil, in between the rubber trees. These vertical shafts sometimes go as far as thirty feet deep and each basketful of soil is lifted to the surface by a crude, but effective, crane arrangement consisting of two bamboo legs and a long bamboo derrick arm with a rope and basket at one end and a counterbalance of large stones tied to the other end.
When a gem-bearing stratum is reached each basketful of soil is placed to one side at the top of the shaft, to be washed and sorted. In some instances a horizontal shaft will be dug to follow the gem-bearing stratum, but as no wooden props or tunnel shores are used the length of these horizontal tunnels is limited by the courage and tenacity of the digger, not to mention his ability to breathe in the tomblike atmosphere. No ladders are provided in the vertical shaft, and entry and exit are effected by bracing the back and hands against one wall and the feet against the opposite side, at the same time exerting the body in a motion that would do credit to James Bond in tightest spot.
There is no natural supply of water for washing the extractions, so the miners pay for this to be brought from the nearby village by water-tank lorries. A small pond about ten feet in diameter and three feet deep is formed near the shaft and the baskets of soil are washed and broken up by members of the group sitting in the pond. As and when the gemstones are found, these are placed in small plastic bags around the perimeter of the pond.
This particular area produces a fair quantity of corundum, most of the crystals being in the form of repeated lamellar twinning. In this form some of the stones can be cut en cabochon to exhibit fine golden six-rayed stars on a dark brown to nearly black background, and on my visit I met Khun Saengroong, a local dealer and cutter, who had just bought a magnificent hexagonal lamellar crystal of star sapphire material weighing 1720 carats. This is of course a rare exception and the average size seldom exceeds 15 carats, and even then only very few of the stones will, when cut, show a well-centered star without the disfiguration of the prominent zone lines which are a feature of the stones from this mine.
The local ‘test’ for rough star sapphire material is to place a drop of water on the stone and to view it from an overhead single light source. In a suitable crystal the ‘star’ will show up clearly when the drop of water is placed in the right position. As it is to be expected, a very great proportion of these opaque corundum crystals show a very poor, or no, star-effect, and these fetch very low prices.
In quantity, the second main gem production of this area is green sapphire, followed by blue/green, yellow/green and more rarely fine blue and yellow sapphire. The hexagonal zoning is easily detected under the lens in a large majority of these stones.
Also associated with the corundum are pyrope garnet (R.I=1.745-1.750) and a fairly large quantity of opaque black stones which take a high polish and are sold both faceted and en cabochon as ‘Thai Jet’. Kenneth Parkinson took ten of these back to the U.K and has since written to tell me that seven of these have a S.G of between 4.1 and 4.2 and with a R.I just visible at the very end of the standard refractometer it seems fairly certain that they are black almandines. The other three stones proved to be black diopside (no star or cat’s eye) with a clear double refraction 1.68 – 1.71. Although this is slightly higher than the normal 1.67 – 1.70, Webster (Gems, 2nd Edition, page 264) notes that the R.I may rise when the material is so dark as to be virtually hedenbergite.
Many jewelers and gem dealers in Bangkok will inform their customers ‘These stones come from our own mine at Chantaburi’, but it is very doubtful whether any of them actually engage in mining themselves, as those who have taken a lease and employed people to dig for them have usually found that somehow their area seems to produce only low-grade stones. The best quality stones will find their way into the market, but not through the lease-holder. The local expression is ‘employ someone to dig and your stone will fly.’
Dealers and middlemen gather at a small group of wooden coffee shops at the fork of two roads leading into the mining area and it is here, in the late afternoon, that the miners bring their daily production for sale.
The existence of these sapphire mines and others in the area producing ruby and zircon, has created a flourishing cutting and setting center in the town of Chantaburi. The standard of work is high, and compared with western prices, cutting costs are very low. A skilled Thai lapidary will be paid about 20 pence for faceting and polishing a zircon of one carat. These low cutting costs have prompted many of the local dealers to import rough gem material for cutting in Thailand and eventual export to the major markets in Europe and the USA. When Kenneth Parkinson and I were in Chantaburi we were shown a parcel of fine blue sapphire crystals recently purchased in Australia. One could not help thinking of the expression ‘bringing coals to Newcastle’.
Although sapphire, ruby and zircon are the principal materials cut at Chantaburi, opal, emerald and other rough is imported for cutting. It is perhaps inevitable that half boules of synthetic corundum are to be seen in many of the gem cutting shops, and, although the majority of dealers will not offer synthetics as anything but what they are, one suspects that a few will be sorely tempted when selling to some of the gullible foreign tourists who are now starting to visit this area.
(via The Journal of Gemmology, Vol.13, No.8, October 1973) J A L Pavitt writes:
Thailand, or Siam as it was formerly named, is a well-known source of sapphire, ruby, star-sapphire and zircon and over the years the skill of Thai lapidaries has advanced to a very high degree, making Bangkok, the capital city, an important center for the supply of cut gemstones.
There are a number of gem mining localities in Thailand, many of them in remote areas, but the mines at Chantaburi (also known as Chantabun), 200 miles from Bangkok, can be reached by car in five and a half hours, and soon after our arrival in Thailand in 1971 my wife and I made our first visit to Khau Ploi Waen, or ‘Hill of the Sapphire Ring’, as this mining area at Chantaburi is named. I have since made further visits, the most recent in January 1973 with Mr Kenneth Parkinson during his two week visit to Thailand.
Chantaburi has a very special place in the history and culture of the Kingdom of Thailand. Situated near the coast, only thirty miles from the border with Cambodia, its inhabitants, although loyal and proud Thai nationals have ethnic origins connecting many of them with the diverse civilizations which existed thousands of years ago between the borders of China and the Mekong Delta. These origins are still evident in the customs, skills, religious and dialects to be found among the people of this fertile eastern region of the Kingdom.
It was at Chantaburi that King Taksin marshaled his forces after the fall of the ancient city of Ayuthaya, and finally defeated and drove out the Burmese invaders. Close to the sapphire mines one can see the rusting cannon and remains of the fortress of King Rama III (1787 – 1851).
The gem-bearing deposits at Khau Ploi Waen are about six miles south of the town of Chantaburi, near the village of Ban Kacha. Past records indicate that in 1850 the Shans and Burmese were extracting sapphires here and that in 1850 a British Company obtained a lease but failed to make a success of the venture. In 1919 the Siam Mining Act came into force and since then mining has been solely in the hands of Thai nationals.
The mining area is privately owned and has been cleared of primary growth and planted with rubber trees, although it is obvious that an income from rubber tapping is of minor importance. A lease to dig for gemstones over an area of one ‘rai’ (approx. 0.4 acre) for one year is granted by the landlords for a fee which may be as high as baht 300000 for high yield areas which have not previously been worked. A lease is usually shared by groups or families and there are said to be some 2000 people mining around Khau Ploi Waen.
The method of extracting the stones is very primitive, as are the tools—a pick, a spade and a rattan basket. A vertical shaft of about four feet in diameter is dug in the red/brown clay soil, in between the rubber trees. These vertical shafts sometimes go as far as thirty feet deep and each basketful of soil is lifted to the surface by a crude, but effective, crane arrangement consisting of two bamboo legs and a long bamboo derrick arm with a rope and basket at one end and a counterbalance of large stones tied to the other end.
When a gem-bearing stratum is reached each basketful of soil is placed to one side at the top of the shaft, to be washed and sorted. In some instances a horizontal shaft will be dug to follow the gem-bearing stratum, but as no wooden props or tunnel shores are used the length of these horizontal tunnels is limited by the courage and tenacity of the digger, not to mention his ability to breathe in the tomblike atmosphere. No ladders are provided in the vertical shaft, and entry and exit are effected by bracing the back and hands against one wall and the feet against the opposite side, at the same time exerting the body in a motion that would do credit to James Bond in tightest spot.
There is no natural supply of water for washing the extractions, so the miners pay for this to be brought from the nearby village by water-tank lorries. A small pond about ten feet in diameter and three feet deep is formed near the shaft and the baskets of soil are washed and broken up by members of the group sitting in the pond. As and when the gemstones are found, these are placed in small plastic bags around the perimeter of the pond.
This particular area produces a fair quantity of corundum, most of the crystals being in the form of repeated lamellar twinning. In this form some of the stones can be cut en cabochon to exhibit fine golden six-rayed stars on a dark brown to nearly black background, and on my visit I met Khun Saengroong, a local dealer and cutter, who had just bought a magnificent hexagonal lamellar crystal of star sapphire material weighing 1720 carats. This is of course a rare exception and the average size seldom exceeds 15 carats, and even then only very few of the stones will, when cut, show a well-centered star without the disfiguration of the prominent zone lines which are a feature of the stones from this mine.
The local ‘test’ for rough star sapphire material is to place a drop of water on the stone and to view it from an overhead single light source. In a suitable crystal the ‘star’ will show up clearly when the drop of water is placed in the right position. As it is to be expected, a very great proportion of these opaque corundum crystals show a very poor, or no, star-effect, and these fetch very low prices.
In quantity, the second main gem production of this area is green sapphire, followed by blue/green, yellow/green and more rarely fine blue and yellow sapphire. The hexagonal zoning is easily detected under the lens in a large majority of these stones.
Also associated with the corundum are pyrope garnet (R.I=1.745-1.750) and a fairly large quantity of opaque black stones which take a high polish and are sold both faceted and en cabochon as ‘Thai Jet’. Kenneth Parkinson took ten of these back to the U.K and has since written to tell me that seven of these have a S.G of between 4.1 and 4.2 and with a R.I just visible at the very end of the standard refractometer it seems fairly certain that they are black almandines. The other three stones proved to be black diopside (no star or cat’s eye) with a clear double refraction 1.68 – 1.71. Although this is slightly higher than the normal 1.67 – 1.70, Webster (Gems, 2nd Edition, page 264) notes that the R.I may rise when the material is so dark as to be virtually hedenbergite.
Many jewelers and gem dealers in Bangkok will inform their customers ‘These stones come from our own mine at Chantaburi’, but it is very doubtful whether any of them actually engage in mining themselves, as those who have taken a lease and employed people to dig for them have usually found that somehow their area seems to produce only low-grade stones. The best quality stones will find their way into the market, but not through the lease-holder. The local expression is ‘employ someone to dig and your stone will fly.’
Dealers and middlemen gather at a small group of wooden coffee shops at the fork of two roads leading into the mining area and it is here, in the late afternoon, that the miners bring their daily production for sale.
The existence of these sapphire mines and others in the area producing ruby and zircon, has created a flourishing cutting and setting center in the town of Chantaburi. The standard of work is high, and compared with western prices, cutting costs are very low. A skilled Thai lapidary will be paid about 20 pence for faceting and polishing a zircon of one carat. These low cutting costs have prompted many of the local dealers to import rough gem material for cutting in Thailand and eventual export to the major markets in Europe and the USA. When Kenneth Parkinson and I were in Chantaburi we were shown a parcel of fine blue sapphire crystals recently purchased in Australia. One could not help thinking of the expression ‘bringing coals to Newcastle’.
Although sapphire, ruby and zircon are the principal materials cut at Chantaburi, opal, emerald and other rough is imported for cutting. It is perhaps inevitable that half boules of synthetic corundum are to be seen in many of the gem cutting shops, and, although the majority of dealers will not offer synthetics as anything but what they are, one suspects that a few will be sorely tempted when selling to some of the gullible foreign tourists who are now starting to visit this area.
Gemmology On A Shoestring
(via The Journal of Gemmology, Vol.10, No.3, July 1966) B W Anderson writes:
Further simple tests
The tests so far suggested have involved only the use of lens and tongs. I am now going to suggest the use of a few very simple ‘extras’ which I feel can be included under our ‘shoestring’ limit, and which will undoubtedly extend quite considerably the scope and certainty of our gem identification. The first extra is the well-known Chelsea filter, which was developed by A Ross Popley from a formula worked out by C J Payne and myself in the early thirties. In knowledgeable hands, and properly used, the filter can be extremely useful. If used without any knowledge of how it functions, it can be quite misleading. Basically, it is a very efficient ‘dichromatic’ filter, transmitting only a narrow band of deep red light and a narrow band of yellow green. Thus, through the filter, a stone can only appear red, green, or a mixture of the two. Filters of this type were originally designed to distinguish between emeralds and pastes or doublets. Emerald, unlike most green stones, both transmits deep red light and emits fluorescent red light when it is brightly illuminated. To see the effect at its best, the stone should be held immediately under a good tungsten light and viewed through the filter held close to the eye. The snags in this simple use of the filter for emerald are that two other green stones, demantoid garnet and fluorspar, may show a reddish residual color when viewed through the filter, that synthetic emeralds appear an even more decisive red than natural emeralds, and that natural emeralds containing enough iron to damp its fluorescence and cause absorption in the deep red do not appear red through the filter. But in my opinion the warning given by the very intense red shown by Chatham synthetic emerald when viewed through the filter is a most useful indication, white its other uses in clearly distinguishing between aquamarine (green appearance) and synthetic blue spinel (orange red) between stained green chalcedony and chrysoprase, etc. serve to add to its value.
Next, quite another kind of filter: Polaroid. This astonishing invention of perhaps the most inventive of modern men, E H Land has placed polarized light, with all its peculiar and revealing properties, within the reach of everyone, and in a form far more compact and convenient than the old Nicol prism. A number of different formulae have been employed in producing the highly dichroic substances in plastic sheets which constitute Polaroid, but the effect in each case is essentially the same—that a ray of light which has passed through such a sheet is vibrating parallel to one direction only—that is, it is completely polarized. Such pieces of polarizing film are capable of far more valuable and fundamental used in the study of gemstones than any color filter can be, and fortunately the material is quite inexpensive; about four shillings per square inch, up to virtually any size required. The most favored type transmits 34% of incident white light, and two pieces of this in the ‘crossed’ position give virtually complete extinction.
One of the most obvious ways of using this material in gem testing is to mount two discs of polaroid in the ‘crossed’ position with a space between them enough to accommodate any gemstone, thereby forming that very sensitive instrument for the detection of double refraction known as the ‘polariscope’. A gadget of this kind can be easily home made, but there are some inexpensive types, carefully designed for convenience in use, which are commercially available. A useful pocket polariscope is one made by Rayner from a design of Dr E Rutland’s, while the same firm make an extremely convenient table model with a built-in light, which leaves ample room for large specimens, either dry or in a cell of suitable liquid; it can also easily accommodate massive pieces of jewelry, stones in necklaces, etc. can be examined, and both hands are free for manipulation.
A gemologist worth his salt will get much more information from such a polariscope than ‘four times light, four times dark—there a doubly refracting crystal’. He will learn to recognize the characteristic types of ‘strain birefringence’ shown by paste imitations (which often show a crude interference cross) by synthetic spinels, with their ‘tabby extinction’ patterns, by diamond, which is never truly isotropic in gem sizes, but shows brilliant interference colors, usually centered on the various points where tiny inclusions can be located, and so on. He will also learn to use a pocket lens to reveal interference figures, at least in the simpler cases, and the sight of a uniaxial figure through the table facet of a ruby or sapphire will give him fairly strong assurance that the stone is a natural one. The unique interference figure of quartz, with its hollow colored center can often provide a quick proof for this mineral. This is beautifully and easily seen in beads of rock crystal or crystal balls, in which the spherical shape makes the figure visible without the use of a lens to provide the ‘conoscope’ effect.
Polaroid can also be used to detect dichroism in gemstones. Two pieces can be set with vibrations at right angles to each other in a pair of old spectacle frames, and a specimen viewed in rapid alternation first with one eye and then the other, often showing a marked change in color, or narrow strips of Polaroid in alternating vibration directions can be counted on a disc of plain glass which can be mounted at one end of a short metal tube with a low power lens at the other. This forms an effective dichroscope for stones that are not too tiny: a dichroic stone viewed through the tube showing alternating stripes of different color or depth of color. Alternatively the Polaroid disc at the end of the tube can be cut into four sectors, the top and bottom sectors transmitting light vibrating, say, north and south, while the left and right hand sectors transmit only light which is vibrating east and west. In testing for dichroism it is best to use daylight reflected from a white surface if possible, and always one should turn both the specimen and the dichroscope tube before deciding on the strength of the dichroism—if there is any.
The next simple and inexpensive aids to gem testing that I want to recommend comprise liquids of various kinds, glass cells of suitable depth and diameter, a glass funnel and some filter papers. These can be used in a number of ways: to enable the color distribution and other internal features of rough or cut stones to be studied with ease: as a rapid test for the density of unmounted stones: and as a means of assessing the refractive index of unknown specimens.
A useful stock to begin with would be two ounces each of methylene iodide, bromoform and bromobenzene, and four ounces of monobromonapthalene, each in screw-top bottles of brown glass.
When stones are immersed in a liquid of similar refractive index the surface reflections and the effects of refraction are largely eliminated and one is able to ‘see into’ the specimens as easily as though they were parallel-sided plates. The ‘frosted’ effect of the surface of rough gem pebbles is also eliminated when they are immersed in a suitable liquid, and lapidaries are well advised to use this means of seeing the color distribution, flaws, etc. of rough gems to enable them to be cut to the best advantage.
Gemologists are familiar with the technique of placing a suspected ruby or sapphire in an immersion cell of liquid before examining it under the microscope, but they may not realize how helpful immersion may be for observations with the naked eye or with a lens. Sapphires in particular can usually be recognized as natural or as synthetic when immersed in bromonapthalene and viewed against a white background. Natural sapphires almost invariably show zones of color with rigidly straight edges, while synthetics slow the well-known curved swathes of color when observed in the correct direction.
A simple and effective means of illumination is to place the stone in its immersion cell on the base of another, inverted glass cell of rather larger size, and direct the light from a bench lamp on to the white blotting paper on which this stands.
For density tests a stone must of course be free from its setting. Granted this, there is no simpler or more decisive way of distinguishing, for instance, between chrysoberyl and quartz cat’s eye, between aquamarine and synthetic spinel, or topaz and yellow quartz, than to put the stone in question into a tube of methylene iodide and seeing whether it floats or sinks. On the whole it is advisable to keep your liquids as pure compounds rather than as mixtures, as in that way their densities and refractive indices remain constant, except for slight variations with temperature. A very useful mixture, however, is one of bromoform diluted with monobromonapthalene until a small clear quartz crystal remains suspended in it. So constant is quartz in its density that this will serve as a virtual identification liquid for any of the transparent quartz gems such as amethyst and citrine. But also it will serve to identify Chatham, Gilson, or Zerfass synthetic emeralds, since the density of these is almost identical to the 2.651 of quartz.
Using liquids as a guide to the refractive index of gemstones has much in common with their used in checking density. In both cases a quite crude test may be all that is required to resolve a doubt, and in both cases quite accurate results can be achieved if this is necessary by taking more time and refining one’s techniques. Even with mounted stones liquids can quickly give useful clues to refractive index. If the small diamonds in a cluster ring, for instance, are suspect, a clear decision can be given if the entire ring be immersed in methylene iodide and the stone viewed with a lens. If the stones are diamond the refractive index is so much higher than that of 1.74 of the liquid that the facets and edges will still appear sharp and clear, while synthetic white spinel and synthetic white sapphire will virtually disappear in this fluid. With loose stones, a very fair idea of their refractivity can be quite quickly and easily obtained by placing the stones table facet down in fair-sized glass cell and pouring in enough monobromonapthalene (R.I = 1.66) to just cove them completely. The cell should be placed on a sheet of white blotting paper and the stones viewed by the light of a single bulb some distance overhead. Those stones with an index higher than that of bromonapthalene will show a distinct dark rim round their periphery, as seen projected on the paper below, and the projection of the facet edges will appear white whereas with stones of lower refractive than the liquid a pale surrounding rim can be seen with the facet edges as dark lines. The degree to which the index of the stone is lower or higher than that of the liquid can be assessed with fair accuracy by noting the width of the dark or pale rim. Unlike the well-known “Becke’ methods for gauging whether an immersed grain is more or less refractive than the liquid, the procedure leaves no doubt at all in the observer’s mind about which has the higher index. A refinement of this simple test is to place the cell containing the immersed stones on a finely-ground glass sheet which is made to act as a bridge between two four-inch blocks of wood or still cardboard. A ‘handbag’ mirror of suitable size is placed at 45º under the cell, enabling the projection onto the glass sheet of the stones, to be seen in detail and in comfort. The effects seen are really very beautiful as well as revealing. If a permanent record is required, a contact photograph can be easily obtained in a dark room by placing the cell containing the stones over a piece of slow film and exposing for a few seconds to an overhead light. Those who are photographers can make use of the narrow beam of light from an enlarger stopped down to f22, and thus obtain very sharp and detailed photographs. Internal features of the stones, in particular color zoning, show up very clearly on such photographs, particularly where the liquid and stones of nearly the same index. I have found that the curved striae in synthetic corundums may be discerned in such photographs even if invisible under the microscope. For this, however, carefully filtered methylene iodide is necessary, and one must be lucky in hitting the right direction for showing the lines. I have only had time to give bare outline of these methods: but anyone seriously interested will find details, with diagrams and photographs, in my book ‘Gem Testing’.
I have included bromobenzene in my short list of useful liquids because this fairly pleasant and inexpensive liquid as a refractive index of 1.56. This makes it an ideal immersion liquid for the critical examination of emerald: it enables the thin rim of dark color to be seen at the edges and corners of the ‘Lechleitner’ type of synthetic emerald, in which a pale, faceted beryl is used as the ‘seed’ on which a thin crystalline layer of synthetic emerald is grown hydrothermally. Quite a few of these stones are now being used in mounted jewelry. The index of bromobenzene is also between that for synthetic emeralds of the Chatham, Gilson and Zerfass types and the indices found in natural emeralds. A good immersion contact photograph of synthetic and natural emeralds immersed in bromobenzene will reveal this.
Two pieces of advice I should like to add because I am so convinced of their importance for any budding gemologist. The first is to start a collection of gemstones, because the ability to compare the appearance and properties of an unknown stone with known samples is of inestimable value in testing. Even a collection of all available synthetics, doublets, pastes, etc. makes a most useful and interesting beginning. For genuine stones the ‘shoestring’ may only permit one to acquire only quite small specimens, or chipped or broken pieces, but one must realize that money spent on any fine specimen is not lost but invested, since the value of these is continuously increasing. The second piece of advice is to have at hand one or two reliable reference books. Two by Robert Webster stand out by reason of their accuracy and comprehensiveness. His ‘Gemologist’s Compendium’ contains all the necessary data gemstones in condensed form, and is very reasonably priced, while his two-volume work ‘Gems’, though expensive, contains such a wealth of information as to be well worth the money for any sizeable firm or jewelers or any serious student of the subject.
A trained gemologist can go a long way in gem identification by intelligent use of a good lens and few simple bits of apparatus, but I must repeat the warning that some of the problems confronting the trade today need more than this for their correct solution: they need all the facilities and skills of a specialized laboratory.
Further simple tests
The tests so far suggested have involved only the use of lens and tongs. I am now going to suggest the use of a few very simple ‘extras’ which I feel can be included under our ‘shoestring’ limit, and which will undoubtedly extend quite considerably the scope and certainty of our gem identification. The first extra is the well-known Chelsea filter, which was developed by A Ross Popley from a formula worked out by C J Payne and myself in the early thirties. In knowledgeable hands, and properly used, the filter can be extremely useful. If used without any knowledge of how it functions, it can be quite misleading. Basically, it is a very efficient ‘dichromatic’ filter, transmitting only a narrow band of deep red light and a narrow band of yellow green. Thus, through the filter, a stone can only appear red, green, or a mixture of the two. Filters of this type were originally designed to distinguish between emeralds and pastes or doublets. Emerald, unlike most green stones, both transmits deep red light and emits fluorescent red light when it is brightly illuminated. To see the effect at its best, the stone should be held immediately under a good tungsten light and viewed through the filter held close to the eye. The snags in this simple use of the filter for emerald are that two other green stones, demantoid garnet and fluorspar, may show a reddish residual color when viewed through the filter, that synthetic emeralds appear an even more decisive red than natural emeralds, and that natural emeralds containing enough iron to damp its fluorescence and cause absorption in the deep red do not appear red through the filter. But in my opinion the warning given by the very intense red shown by Chatham synthetic emerald when viewed through the filter is a most useful indication, white its other uses in clearly distinguishing between aquamarine (green appearance) and synthetic blue spinel (orange red) between stained green chalcedony and chrysoprase, etc. serve to add to its value.
Next, quite another kind of filter: Polaroid. This astonishing invention of perhaps the most inventive of modern men, E H Land has placed polarized light, with all its peculiar and revealing properties, within the reach of everyone, and in a form far more compact and convenient than the old Nicol prism. A number of different formulae have been employed in producing the highly dichroic substances in plastic sheets which constitute Polaroid, but the effect in each case is essentially the same—that a ray of light which has passed through such a sheet is vibrating parallel to one direction only—that is, it is completely polarized. Such pieces of polarizing film are capable of far more valuable and fundamental used in the study of gemstones than any color filter can be, and fortunately the material is quite inexpensive; about four shillings per square inch, up to virtually any size required. The most favored type transmits 34% of incident white light, and two pieces of this in the ‘crossed’ position give virtually complete extinction.
One of the most obvious ways of using this material in gem testing is to mount two discs of polaroid in the ‘crossed’ position with a space between them enough to accommodate any gemstone, thereby forming that very sensitive instrument for the detection of double refraction known as the ‘polariscope’. A gadget of this kind can be easily home made, but there are some inexpensive types, carefully designed for convenience in use, which are commercially available. A useful pocket polariscope is one made by Rayner from a design of Dr E Rutland’s, while the same firm make an extremely convenient table model with a built-in light, which leaves ample room for large specimens, either dry or in a cell of suitable liquid; it can also easily accommodate massive pieces of jewelry, stones in necklaces, etc. can be examined, and both hands are free for manipulation.
A gemologist worth his salt will get much more information from such a polariscope than ‘four times light, four times dark—there a doubly refracting crystal’. He will learn to recognize the characteristic types of ‘strain birefringence’ shown by paste imitations (which often show a crude interference cross) by synthetic spinels, with their ‘tabby extinction’ patterns, by diamond, which is never truly isotropic in gem sizes, but shows brilliant interference colors, usually centered on the various points where tiny inclusions can be located, and so on. He will also learn to use a pocket lens to reveal interference figures, at least in the simpler cases, and the sight of a uniaxial figure through the table facet of a ruby or sapphire will give him fairly strong assurance that the stone is a natural one. The unique interference figure of quartz, with its hollow colored center can often provide a quick proof for this mineral. This is beautifully and easily seen in beads of rock crystal or crystal balls, in which the spherical shape makes the figure visible without the use of a lens to provide the ‘conoscope’ effect.
Polaroid can also be used to detect dichroism in gemstones. Two pieces can be set with vibrations at right angles to each other in a pair of old spectacle frames, and a specimen viewed in rapid alternation first with one eye and then the other, often showing a marked change in color, or narrow strips of Polaroid in alternating vibration directions can be counted on a disc of plain glass which can be mounted at one end of a short metal tube with a low power lens at the other. This forms an effective dichroscope for stones that are not too tiny: a dichroic stone viewed through the tube showing alternating stripes of different color or depth of color. Alternatively the Polaroid disc at the end of the tube can be cut into four sectors, the top and bottom sectors transmitting light vibrating, say, north and south, while the left and right hand sectors transmit only light which is vibrating east and west. In testing for dichroism it is best to use daylight reflected from a white surface if possible, and always one should turn both the specimen and the dichroscope tube before deciding on the strength of the dichroism—if there is any.
The next simple and inexpensive aids to gem testing that I want to recommend comprise liquids of various kinds, glass cells of suitable depth and diameter, a glass funnel and some filter papers. These can be used in a number of ways: to enable the color distribution and other internal features of rough or cut stones to be studied with ease: as a rapid test for the density of unmounted stones: and as a means of assessing the refractive index of unknown specimens.
A useful stock to begin with would be two ounces each of methylene iodide, bromoform and bromobenzene, and four ounces of monobromonapthalene, each in screw-top bottles of brown glass.
When stones are immersed in a liquid of similar refractive index the surface reflections and the effects of refraction are largely eliminated and one is able to ‘see into’ the specimens as easily as though they were parallel-sided plates. The ‘frosted’ effect of the surface of rough gem pebbles is also eliminated when they are immersed in a suitable liquid, and lapidaries are well advised to use this means of seeing the color distribution, flaws, etc. of rough gems to enable them to be cut to the best advantage.
Gemologists are familiar with the technique of placing a suspected ruby or sapphire in an immersion cell of liquid before examining it under the microscope, but they may not realize how helpful immersion may be for observations with the naked eye or with a lens. Sapphires in particular can usually be recognized as natural or as synthetic when immersed in bromonapthalene and viewed against a white background. Natural sapphires almost invariably show zones of color with rigidly straight edges, while synthetics slow the well-known curved swathes of color when observed in the correct direction.
A simple and effective means of illumination is to place the stone in its immersion cell on the base of another, inverted glass cell of rather larger size, and direct the light from a bench lamp on to the white blotting paper on which this stands.
For density tests a stone must of course be free from its setting. Granted this, there is no simpler or more decisive way of distinguishing, for instance, between chrysoberyl and quartz cat’s eye, between aquamarine and synthetic spinel, or topaz and yellow quartz, than to put the stone in question into a tube of methylene iodide and seeing whether it floats or sinks. On the whole it is advisable to keep your liquids as pure compounds rather than as mixtures, as in that way their densities and refractive indices remain constant, except for slight variations with temperature. A very useful mixture, however, is one of bromoform diluted with monobromonapthalene until a small clear quartz crystal remains suspended in it. So constant is quartz in its density that this will serve as a virtual identification liquid for any of the transparent quartz gems such as amethyst and citrine. But also it will serve to identify Chatham, Gilson, or Zerfass synthetic emeralds, since the density of these is almost identical to the 2.651 of quartz.
Using liquids as a guide to the refractive index of gemstones has much in common with their used in checking density. In both cases a quite crude test may be all that is required to resolve a doubt, and in both cases quite accurate results can be achieved if this is necessary by taking more time and refining one’s techniques. Even with mounted stones liquids can quickly give useful clues to refractive index. If the small diamonds in a cluster ring, for instance, are suspect, a clear decision can be given if the entire ring be immersed in methylene iodide and the stone viewed with a lens. If the stones are diamond the refractive index is so much higher than that of 1.74 of the liquid that the facets and edges will still appear sharp and clear, while synthetic white spinel and synthetic white sapphire will virtually disappear in this fluid. With loose stones, a very fair idea of their refractivity can be quite quickly and easily obtained by placing the stones table facet down in fair-sized glass cell and pouring in enough monobromonapthalene (R.I = 1.66) to just cove them completely. The cell should be placed on a sheet of white blotting paper and the stones viewed by the light of a single bulb some distance overhead. Those stones with an index higher than that of bromonapthalene will show a distinct dark rim round their periphery, as seen projected on the paper below, and the projection of the facet edges will appear white whereas with stones of lower refractive than the liquid a pale surrounding rim can be seen with the facet edges as dark lines. The degree to which the index of the stone is lower or higher than that of the liquid can be assessed with fair accuracy by noting the width of the dark or pale rim. Unlike the well-known “Becke’ methods for gauging whether an immersed grain is more or less refractive than the liquid, the procedure leaves no doubt at all in the observer’s mind about which has the higher index. A refinement of this simple test is to place the cell containing the immersed stones on a finely-ground glass sheet which is made to act as a bridge between two four-inch blocks of wood or still cardboard. A ‘handbag’ mirror of suitable size is placed at 45º under the cell, enabling the projection onto the glass sheet of the stones, to be seen in detail and in comfort. The effects seen are really very beautiful as well as revealing. If a permanent record is required, a contact photograph can be easily obtained in a dark room by placing the cell containing the stones over a piece of slow film and exposing for a few seconds to an overhead light. Those who are photographers can make use of the narrow beam of light from an enlarger stopped down to f22, and thus obtain very sharp and detailed photographs. Internal features of the stones, in particular color zoning, show up very clearly on such photographs, particularly where the liquid and stones of nearly the same index. I have found that the curved striae in synthetic corundums may be discerned in such photographs even if invisible under the microscope. For this, however, carefully filtered methylene iodide is necessary, and one must be lucky in hitting the right direction for showing the lines. I have only had time to give bare outline of these methods: but anyone seriously interested will find details, with diagrams and photographs, in my book ‘Gem Testing’.
I have included bromobenzene in my short list of useful liquids because this fairly pleasant and inexpensive liquid as a refractive index of 1.56. This makes it an ideal immersion liquid for the critical examination of emerald: it enables the thin rim of dark color to be seen at the edges and corners of the ‘Lechleitner’ type of synthetic emerald, in which a pale, faceted beryl is used as the ‘seed’ on which a thin crystalline layer of synthetic emerald is grown hydrothermally. Quite a few of these stones are now being used in mounted jewelry. The index of bromobenzene is also between that for synthetic emeralds of the Chatham, Gilson and Zerfass types and the indices found in natural emeralds. A good immersion contact photograph of synthetic and natural emeralds immersed in bromobenzene will reveal this.
Two pieces of advice I should like to add because I am so convinced of their importance for any budding gemologist. The first is to start a collection of gemstones, because the ability to compare the appearance and properties of an unknown stone with known samples is of inestimable value in testing. Even a collection of all available synthetics, doublets, pastes, etc. makes a most useful and interesting beginning. For genuine stones the ‘shoestring’ may only permit one to acquire only quite small specimens, or chipped or broken pieces, but one must realize that money spent on any fine specimen is not lost but invested, since the value of these is continuously increasing. The second piece of advice is to have at hand one or two reliable reference books. Two by Robert Webster stand out by reason of their accuracy and comprehensiveness. His ‘Gemologist’s Compendium’ contains all the necessary data gemstones in condensed form, and is very reasonably priced, while his two-volume work ‘Gems’, though expensive, contains such a wealth of information as to be well worth the money for any sizeable firm or jewelers or any serious student of the subject.
A trained gemologist can go a long way in gem identification by intelligent use of a good lens and few simple bits of apparatus, but I must repeat the warning that some of the problems confronting the trade today need more than this for their correct solution: they need all the facilities and skills of a specialized laboratory.
Sunday, July 08, 2007
Benitoite
Chemistry: Barium titanium silicate
Crystal system: Trigonal; crystal class ditrigonal bipyramidal; commonly as trigonal bipyramids.
Color: Transparent to translucent; blue (most common), violet blue, colorless, pink (rare).
Hardness: 6.5
Cleavage: None; Fracture: conchoidal.
Specific gravity: 3.65
Refractive index: 1.75 – 1.80; Uniaxial positive; 0.047; strong doubling.
Luster: Vitreous – sub-adamantine.
Dispersion: High (masked by body color).
Dichroism: Strong (blue and colorless).
Occurrence: San Benito County, California, U.S.A.
Notes
Discovered in 1906; Collectors stone; Looks like sapphire, but pronounced DR, dichroism and high dispersion, luminescence, and lower SG help in separation; bright blue in short wave; faceted, usually round brilliants to display dispersion.
Crystal system: Trigonal; crystal class ditrigonal bipyramidal; commonly as trigonal bipyramids.
Color: Transparent to translucent; blue (most common), violet blue, colorless, pink (rare).
Hardness: 6.5
Cleavage: None; Fracture: conchoidal.
Specific gravity: 3.65
Refractive index: 1.75 – 1.80; Uniaxial positive; 0.047; strong doubling.
Luster: Vitreous – sub-adamantine.
Dispersion: High (masked by body color).
Dichroism: Strong (blue and colorless).
Occurrence: San Benito County, California, U.S.A.
Notes
Discovered in 1906; Collectors stone; Looks like sapphire, but pronounced DR, dichroism and high dispersion, luminescence, and lower SG help in separation; bright blue in short wave; faceted, usually round brilliants to display dispersion.
U.S. Government Watchdog Demands Better Governmental Controls Over Kimberley Process
Chaim Even-Zohar writes about the GAO, the U.S governmental watchdog + its report to the U.S Congress on Kimberly Process Certification Scheme (KPCS) + other viewpoints @ http://www.idexonline.com/portal_FullEditorial.asp?TextSearch=&KeyMatch=0&id=26271
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