(via Australian Gemmologist, Volume 23, No.2, April – June 2007) Alan Hodgkinson writes:
The author describes how liquids such as sodium polytungstate and glycerine can be used practically to discriminate natural opals from synthetic and all-plastic imitation opals.
A typical gem quality opal has a water content of 6-10 percent and this is subject to both evaporation and drying out. In contrast, however, some opals are extremely hygroscopic, and upon immersion in water there can be considerable intake. This not only alters the weight and therefore the density of opal, but also can alter its appearance. As a consequence of this water intake, the whole stone looks more valuable. Also note how the opal has temporarily darkened where the water intake has concentrated.
Natural opal weight (before): 16.19ct
Natural opal weight (after): 18.10 ct
Obviously, any attempt at determining the specific gravity of such an opal, by hydrostatic or beam balance method would upset by its water intake. However, a useful response for estimating this opals specific gravity can be gained by the immediate reaction of the opal to immersion in sodium polytungstate. Webster gives the S.G for white and black and opal as near 2.1. Bear in mind, this density range is only significant when there is no matrix present. Please remember that the latter feature itself a pointer to a natural opal identity, unless the opal is part of a composite imitation. Those man-made opals with a 20 to 30 percent polymer infilling have a decidedly lower S.G in the region of 1.8 – 1.9, while those white synthetic opals that use zirconia as a sphere binder have a slightly raised S.G in the region of 2.2.
Suspension in sodium polytungstate
It is fairly simple matter to suspend an opal in the harmless medium, sodium polytungstate. This compound was demonstrated to the author in 1979 by Dr W W Hanneman. When pure, sodium polytungstate’s S.G is 2.8. Adding distilled water lowers its density. In contrast, heating the liquid evaporates and therefore raises its density. Once the opal is suspended, the R.I of the liquid can be confirmed by refractometer. This can then be read off against a straight line graph compiled by Hanneman which will indicate the approximate density of the opal. The alternative is to use known control stones (natural and synthetic) and compare the buoyancy or heaviness of the test opal against the behavior of the controls.
Glycerin as a heavy liquid
Glycerin is not harmful, washes off immediately with cold water, is not expensive, and might be found useful to separate opals from the all-plastic opal simulants, as pointed out by Emmanuel Fritsch. This viscous liquid has a density of 1.26 in which all imitation plastic opals will float. In contrast, all opal, whether natural or synthetic, will sink. Even those man-made opals which contain silica spheres but which have a substantial plastic binder, will sink. Apart from the plastic opal imitations, all of the above sink at variable speeds—dependent on size, and not density.
Thursday, May 24, 2007
Wednesday, May 23, 2007
Mexican Turquoise
It has been reported that new production of turquoise from a large open-pit copper mine, about 110 km south of the U.S border from Douglas, Arizona is the new source. The Nacozari turquoise is found in nodules; though chalky much of the material is hard enough with good color. The rough pieces show inclusions of pyrite, quartz, and mica. Most materials are fashioned as cabochon or free form. The materials may be stabilized to improve color.
Burmese Pezzottaite
It has been reported that after Madagascar and Afghanistan, Khatchel, near Molo, in the Momeik area of Burma's Mogok district is the new source for pezzottaite. The crystals are tabular with colors ranging from pinkish orange to purplish pink.
Andes Jade
Andes Jade is the marketing name for serpentinite from central-western Argentina. The material is translucent to opaque with colors ranging from light to dark green, to bluish green to black. Analytical tests indicate a mixture of antigorite and lizardite with magnesite and magnetite. Some specimens are magnetic. The material is used for carvings; good ones may be cut as cabochons.
How Color Names Developed
Here is an interesting concept on color communication for colored stone dealers, gemologists, jewelers and consumers alike.
Methuen writes:
The purpose of a color name is to communicate the appearance of a given color or to enable us to think in color. Thus the color name must be so characteristic of the color’s appearance that it is readily understood by others. Since our environment is the source of colors, it is here that we must look for objects of typical colors, objects for which we already have names and which can be used to designate a characteristic appearance. Such was the case with the words blood and red, the oldest color name found in most languages. White and black are also of ancient origin and often derived from the concepts of light and dark symbolized by day and night. From the Sanskrit candra (light) came the Latin candidus (white). The Russian belyi (white) is derived from the root bhe (to lighten). The English white is derived from the Germanic xwitaz, which is related to the Russian svet (light). Similarly, black and dark share common origins. The German Schwarz (black) is related to the Nordic sortna (to darken), the Latin suasum (a dark place), and the archaic English swart, which means darkness and black (hence swarthy).
The word for yellow is also of early origin in many languages, usually derived from the names for fruits, straw, gold, fire or bile (the German galle, the Italian giallo). The words for green and blue developed at a later date, perhaps because the materials necessary to form these pigments were not as readily available. Green is naturally related to the phenomenon of growth and greenery. Blue, strangely enough, is often derived from the word for pale or yellow, perhaps because the sky often is a pale blue or even yellowish.
Among the oldest color names, those for red, yellow, green, blue, white, black, correspond with the basic concepts discussed previously. Here we shall describe these as basic names of the first order. Names of the second order are words such as beige, blonde, grey, brown, golden, lilac, magenta, olive, orange, pale, purple, rose, ruby, turquoise and violet, that is, color names which are independent words and characterize more or less specific colors or color areas.
When basic color names are combined with one another, they designate intermediate colors such as yellow green, blue black, golden blonde. The basic color names and their combinations will be grouped together under the term general color names. As the need for more refined color descriptions arose, the general color names were varied by the addition of modifiers resulting in terms such as light blue, pastel green, deep black. The addition of suffixes, such as the ish in yellowish, can be useful in describing the changes which take place in a gradual color shift, for example: green, bluish green, blue green, green blue, greenish blue, blue.
Another, larger group of color names are directly derived from specific elements in our environment. In addition to the words for the specific elements or objects which identify colors, the terms in this group usually include a basic color name. Below are some examples of color names which belong to this group:
1. Color names derived from plants: apricot (yellow), lemon yellow, grass green, hazel, rose red.
2. Color names derived from minerals and metals: alabaster, amethyst (violet), copper (red), malachite green, platinum blonde, turquoise blue.
3. Color names derived from man-made products: chocolate (brown), faience blue, bottle green, wine red.
4. Color names derived from fauna: beaver, canary yellow, mouse grey, fox, butterfly blue.
5. Color names derived from geographic names: Berlin blue, Copenhagen blue, Naples yellow, Pompeian red, Spanish green.
6. Color names derived from natural phenomena: aurora, spring green, sky blue, fire red, fog.
7. Color names derived from miscellaneous subjects: calypso (red), infra-red.
The importance of the basic color names and particularly those of the first order is evident from the frequency with which they occur in color descriptions. About one half of color descriptions contain a basic name of the first order and about one quarter a secondary basic name; in addition, combinations of both occupy about one sixth of such descriptions.
Thus far we have considered color names of one or two words, for example, blue, yellow grey, light red, deep black (yellow plus grey, light plus red, deep plus black). Color names with three or more parts are impractical. Four-part names such as light blue green grey are exceedingly difficult to visualize; even a three-part name such as grey brown red can prove awkward in usage.
In everyday speech, however, we often use auxiliary words before the proper color name; for example, a radiant orange red, a strong blue violet, a dark blue green, a warm red brown. These auxiliary words specify a slight variation of the color name proper. With time, part of the color name proper may itself become an auxiliary word; the name grey violet, for example, became more commonly known as grayish violet, and blue green also became known as bluish green. There are too many auxiliary words, such a strong and deep, to include in the dictionary. If well chosen, however, their meaning can be easily understood.
The method of forming color names outlined here obviously permits the formation of an almost unlimited number of color names, many of which would prove superfluous. Although such a flood of names is impractical, constantly changing fashions demand an ever increasing variety of color names—often in such fantastic combinations that a name becomes vague or meaningless; when it does have a meaning, this may change from year to year.
Methuen writes:
The purpose of a color name is to communicate the appearance of a given color or to enable us to think in color. Thus the color name must be so characteristic of the color’s appearance that it is readily understood by others. Since our environment is the source of colors, it is here that we must look for objects of typical colors, objects for which we already have names and which can be used to designate a characteristic appearance. Such was the case with the words blood and red, the oldest color name found in most languages. White and black are also of ancient origin and often derived from the concepts of light and dark symbolized by day and night. From the Sanskrit candra (light) came the Latin candidus (white). The Russian belyi (white) is derived from the root bhe (to lighten). The English white is derived from the Germanic xwitaz, which is related to the Russian svet (light). Similarly, black and dark share common origins. The German Schwarz (black) is related to the Nordic sortna (to darken), the Latin suasum (a dark place), and the archaic English swart, which means darkness and black (hence swarthy).
The word for yellow is also of early origin in many languages, usually derived from the names for fruits, straw, gold, fire or bile (the German galle, the Italian giallo). The words for green and blue developed at a later date, perhaps because the materials necessary to form these pigments were not as readily available. Green is naturally related to the phenomenon of growth and greenery. Blue, strangely enough, is often derived from the word for pale or yellow, perhaps because the sky often is a pale blue or even yellowish.
Among the oldest color names, those for red, yellow, green, blue, white, black, correspond with the basic concepts discussed previously. Here we shall describe these as basic names of the first order. Names of the second order are words such as beige, blonde, grey, brown, golden, lilac, magenta, olive, orange, pale, purple, rose, ruby, turquoise and violet, that is, color names which are independent words and characterize more or less specific colors or color areas.
When basic color names are combined with one another, they designate intermediate colors such as yellow green, blue black, golden blonde. The basic color names and their combinations will be grouped together under the term general color names. As the need for more refined color descriptions arose, the general color names were varied by the addition of modifiers resulting in terms such as light blue, pastel green, deep black. The addition of suffixes, such as the ish in yellowish, can be useful in describing the changes which take place in a gradual color shift, for example: green, bluish green, blue green, green blue, greenish blue, blue.
Another, larger group of color names are directly derived from specific elements in our environment. In addition to the words for the specific elements or objects which identify colors, the terms in this group usually include a basic color name. Below are some examples of color names which belong to this group:
1. Color names derived from plants: apricot (yellow), lemon yellow, grass green, hazel, rose red.
2. Color names derived from minerals and metals: alabaster, amethyst (violet), copper (red), malachite green, platinum blonde, turquoise blue.
3. Color names derived from man-made products: chocolate (brown), faience blue, bottle green, wine red.
4. Color names derived from fauna: beaver, canary yellow, mouse grey, fox, butterfly blue.
5. Color names derived from geographic names: Berlin blue, Copenhagen blue, Naples yellow, Pompeian red, Spanish green.
6. Color names derived from natural phenomena: aurora, spring green, sky blue, fire red, fog.
7. Color names derived from miscellaneous subjects: calypso (red), infra-red.
The importance of the basic color names and particularly those of the first order is evident from the frequency with which they occur in color descriptions. About one half of color descriptions contain a basic name of the first order and about one quarter a secondary basic name; in addition, combinations of both occupy about one sixth of such descriptions.
Thus far we have considered color names of one or two words, for example, blue, yellow grey, light red, deep black (yellow plus grey, light plus red, deep plus black). Color names with three or more parts are impractical. Four-part names such as light blue green grey are exceedingly difficult to visualize; even a three-part name such as grey brown red can prove awkward in usage.
In everyday speech, however, we often use auxiliary words before the proper color name; for example, a radiant orange red, a strong blue violet, a dark blue green, a warm red brown. These auxiliary words specify a slight variation of the color name proper. With time, part of the color name proper may itself become an auxiliary word; the name grey violet, for example, became more commonly known as grayish violet, and blue green also became known as bluish green. There are too many auxiliary words, such a strong and deep, to include in the dictionary. If well chosen, however, their meaning can be easily understood.
The method of forming color names outlined here obviously permits the formation of an almost unlimited number of color names, many of which would prove superfluous. Although such a flood of names is impractical, constantly changing fashions demand an ever increasing variety of color names—often in such fantastic combinations that a name becomes vague or meaningless; when it does have a meaning, this may change from year to year.
Tuesday, May 22, 2007
World's Best Presentation Contest Winners
Slideshare.net announced the winners of the World's Best Presentation Contest:
Winners (chose by judges)
1. ShiftHappens by Jbrenman
2. Meet Henry by Chereemoore
3. Sustainable Food Lab by Chrislandry
People’s Choice Winners
1. PaniPuri--An Introduction by Thakkar
2. ShiftHappens by Jbrenman
3. Meet Henry by Chereeemoore
Tips for good presentation:
1. Big Fonts
2. Big Graphics
3. Good Story
Winners (chose by judges)
1. ShiftHappens by Jbrenman
2. Meet Henry by Chereemoore
3. Sustainable Food Lab by Chrislandry
People’s Choice Winners
1. PaniPuri--An Introduction by Thakkar
2. ShiftHappens by Jbrenman
3. Meet Henry by Chereeemoore
Tips for good presentation:
1. Big Fonts
2. Big Graphics
3. Good Story
The Trapiche Growth Phenomenon
There are various interpretations on the trapiche growth phenomena in colored stones. In the past experts got confused with the unusual growth pattern to twinning. The unusual growth pattern is usually visible in beryl, variety emerald, (Muzo, La Pena, Coscuez) and corundum, variety ruby and sapphire (Burma(Mong Hsu) and Vietnam). One theory is that the phenomenon in beryl and corundum are due to skeletal or dendritic growth, where edges and corners tend to grow much faster than the faces of a crystal. Another interpretation on the phenomenon in trapiche rubies and sapphires from Burma and Vietnam are due to skeletal growth followed by layer-by-layer growth. To make a long story short, rapid growth and changes in the growth conditions are believed to be the origin of the unusual patterns in beryl and corundum. The trapiche phenomenon is also seen in andalusite, tourmaline, and quartz. For now there are no nomenclatures for fixed star patterns that are being offered as trapiche in the gem market.
A Question Of Origin: A Different View
(via Gemological Digest, Vol.3, No.1, 1990) Charles A Schiffman writes:
Relating a source to the presence of certain inclusions or other physical properties is not a recent idea, but goes back to the last century. A good example in the literature is found in Max Bauer’s Edelsteinkunde, 3rd edition, Tauchnitz Verlag, 1896/1932; the author points out the great importance of inclusions in determining origins (page 499) and goes on about their description.
In more recent times, in the Handbook Of Gem Identification by R.T.Liddicoat, 11th edition, GIA, 1981, typical inclusions related to sources are quoted (pages 88-89, 95-96).
Inclusions As A Means Of Gemstone Identification by E.Gubelin, GIA, 1953, has been the standard course book at the GIA on the matter of inclusions related to source.
That gemstones of certain sources were highly favored long ago is a historical fact, independent from gemology.
This writer recalls one merchant who was proud because he dealt only in Burma rubies that he bought directly from Burma, following requests from customers in the jewelry trade, and he was not an isolated case.
The trade has a strong demand for origins that is documented by the Definitions of the CIBJO (published about 1968) in Europe, a body grouping a majority of dealers and retailers. The following is extracted from CIBJO:
Art.2: Designation of color and place of origin (a) names of gem localities used heretofore to describe the color of gemstones, cannot be used any longer (b) indications of place of origin can be only added if the origin is known and can actually be corroborated. This proof may be given either by physical properties, or by the stone’s inclusions, in so far as they are characteristic of a definite area.
Obviously there are limitations to finding out origins, i.e the absence of characteristic features. For this reason, the origin of many specimens of such gemstones as garnet, tourmaline and quart cannot be found.
An interesting point is that considering these limitations, where no origin may be objectively ascertained, a lab sometimes faces little understanding in trade circles.
Answering the demand from the market for general testing and for determination of origin, Gubelin Laboratory extended its activities (previously only for the company’s own needs) to outside inquiries in the late 1960’s.
Time has not stood still since then. Investigation has become a challenge to go beyond routine methods, using more complex instrumentation. Even so, some limitations will remain in the feasibility of this complex and difficult task, in an effort to base it on objective methods.
On the other side, the gemologist familiar with this field is conscious of the big commercial assets involved by people chasing the rare and exceptional items. This attitude is a very human one, so that it is hard to believe that interested parties will just forget about demanding origin information.
Showing different aspect of the question in this magazine will hopefully lead to a better understanding of the parties concerned, to improve their cooperation and contribute to solving pending questions.
Relating a source to the presence of certain inclusions or other physical properties is not a recent idea, but goes back to the last century. A good example in the literature is found in Max Bauer’s Edelsteinkunde, 3rd edition, Tauchnitz Verlag, 1896/1932; the author points out the great importance of inclusions in determining origins (page 499) and goes on about their description.
In more recent times, in the Handbook Of Gem Identification by R.T.Liddicoat, 11th edition, GIA, 1981, typical inclusions related to sources are quoted (pages 88-89, 95-96).
Inclusions As A Means Of Gemstone Identification by E.Gubelin, GIA, 1953, has been the standard course book at the GIA on the matter of inclusions related to source.
That gemstones of certain sources were highly favored long ago is a historical fact, independent from gemology.
This writer recalls one merchant who was proud because he dealt only in Burma rubies that he bought directly from Burma, following requests from customers in the jewelry trade, and he was not an isolated case.
The trade has a strong demand for origins that is documented by the Definitions of the CIBJO (published about 1968) in Europe, a body grouping a majority of dealers and retailers. The following is extracted from CIBJO:
Art.2: Designation of color and place of origin (a) names of gem localities used heretofore to describe the color of gemstones, cannot be used any longer (b) indications of place of origin can be only added if the origin is known and can actually be corroborated. This proof may be given either by physical properties, or by the stone’s inclusions, in so far as they are characteristic of a definite area.
Obviously there are limitations to finding out origins, i.e the absence of characteristic features. For this reason, the origin of many specimens of such gemstones as garnet, tourmaline and quart cannot be found.
An interesting point is that considering these limitations, where no origin may be objectively ascertained, a lab sometimes faces little understanding in trade circles.
Answering the demand from the market for general testing and for determination of origin, Gubelin Laboratory extended its activities (previously only for the company’s own needs) to outside inquiries in the late 1960’s.
Time has not stood still since then. Investigation has become a challenge to go beyond routine methods, using more complex instrumentation. Even so, some limitations will remain in the feasibility of this complex and difficult task, in an effort to base it on objective methods.
On the other side, the gemologist familiar with this field is conscious of the big commercial assets involved by people chasing the rare and exceptional items. This attitude is a very human one, so that it is hard to believe that interested parties will just forget about demanding origin information.
Showing different aspect of the question in this magazine will hopefully lead to a better understanding of the parties concerned, to improve their cooperation and contribute to solving pending questions.
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