David Federman writes:
Australia was to sapphire in the 1980s what South Africa was to diamonds for more than a century. Still, no one felt blue when Aussie indigos disappeared under a blanket of Madagascar's dressier blues in the 1990s.
The new African corundum boasted color that begged comparison to that of Sri Lankan goods—even on occasion Kashmir classics. Australia's material, on the other hand, was lucky to inspire comparisons to dark but sometimes handsome Cambodian and Thai stones. No wonder Australia came and went as a corundum colossus and no one shed a tear.
More info @ http://archives.modernjeweler.com/publication/article.jsp?pubId=1&id=193
Tuesday, January 09, 2007
The Colored Stone Story
What are colored stones?
Colored stones refer to any gem other than diamond. Some diamonds are colored, but colored diamonds are not considered colored stones.
What are the attributes of colored stones?
A colored stone possesses any of these three key characteristics: beauty, durability and rarity. An ideal colored stone has all three characteristics.
Why do colored stones look the way they do?
A number of factors contribute to the overall appearance of colored stones. These include color, transparency, lustre, brilliance, dispersion, scintillation, clarity and cut.
What is the difference between precious stones and semi-precious stones?
In the past, the term ‘precious stone’ referred to diamond, emerald, ruby and sapphire, while ‘semi-precious stone’ were anything other than these. Then, precious and semi-precious stones indicated relative value – the former being more valuable. Today, these descriptions have little meaning in terms of value, since all colored stones come in a variety of qualities. For example, a low quality ruby may sell for as little as USD1 per carat, while a good quality demantoid garnet may fetch USD1,000 per carat.
Are colored stones durable?
In order to wear well and retain attractiveness colored stones should be durable. Durability includes hardness and toughness. Resistance to scratching is referred to as hardness. Some colored stones are soft and may be easily scratched, thereby losing their attractiveness. Ideally a colored stone should be harder than dust particles, which may be fine particles of quartz (hardness=7) found in the air. Colored stones with hardness greater than quartz include ruby, sapphire, topaz, tourmaline, spinel and garnet. These are the most valuable. Resistance to breakage is referred to as toughness, and this depends on colored stone’s structure. Extremely tough gems are usually aggregates such as jadeite and nephrite.
Is there a relationship between quality and price?
In a perfect world price would directly relate to quality, weight and rarity. The real world is far from perfect. Market factors may have an impact on prices, as does quality.
What is colored stone treatment?
Colored stone treatment is a common term. It describes different ways to improve the beauty of colored stones. Colored stones have been enhanced for thousands of years. The truth is there isn’t enough top quality colored stones being mined to meet demand. Enhancement increases supply, ensuring a constant flow of colored stones which we can afford and enjoy. If in doubt about a colored stone, always consult a reputed gem testing laboratory.
Colored stones refer to any gem other than diamond. Some diamonds are colored, but colored diamonds are not considered colored stones.
What are the attributes of colored stones?
A colored stone possesses any of these three key characteristics: beauty, durability and rarity. An ideal colored stone has all three characteristics.
Why do colored stones look the way they do?
A number of factors contribute to the overall appearance of colored stones. These include color, transparency, lustre, brilliance, dispersion, scintillation, clarity and cut.
What is the difference between precious stones and semi-precious stones?
In the past, the term ‘precious stone’ referred to diamond, emerald, ruby and sapphire, while ‘semi-precious stone’ were anything other than these. Then, precious and semi-precious stones indicated relative value – the former being more valuable. Today, these descriptions have little meaning in terms of value, since all colored stones come in a variety of qualities. For example, a low quality ruby may sell for as little as USD1 per carat, while a good quality demantoid garnet may fetch USD1,000 per carat.
Are colored stones durable?
In order to wear well and retain attractiveness colored stones should be durable. Durability includes hardness and toughness. Resistance to scratching is referred to as hardness. Some colored stones are soft and may be easily scratched, thereby losing their attractiveness. Ideally a colored stone should be harder than dust particles, which may be fine particles of quartz (hardness=7) found in the air. Colored stones with hardness greater than quartz include ruby, sapphire, topaz, tourmaline, spinel and garnet. These are the most valuable. Resistance to breakage is referred to as toughness, and this depends on colored stone’s structure. Extremely tough gems are usually aggregates such as jadeite and nephrite.
Is there a relationship between quality and price?
In a perfect world price would directly relate to quality, weight and rarity. The real world is far from perfect. Market factors may have an impact on prices, as does quality.
What is colored stone treatment?
Colored stone treatment is a common term. It describes different ways to improve the beauty of colored stones. Colored stones have been enhanced for thousands of years. The truth is there isn’t enough top quality colored stones being mined to meet demand. Enhancement increases supply, ensuring a constant flow of colored stones which we can afford and enjoy. If in doubt about a colored stone, always consult a reputed gem testing laboratory.
The Chelsea Color Filter
The Chelsea Color Filter was first developed by the Gem Testing Laboratory of the London Chamber of Commerce (now GAGTL/Gem-A) in 1934, and was first used at the Chelsea College of Science and Technology as an aid in the separation of emerald from its simulants. It was originally called an emerald filter.
The Chelsea filter consists of a combination of two gelatin filters that pass wavelengths in the deep red and in the yellow green. To obtain best results, stones should be viewed under a strong artificial light with the filter held close to the eye so as to cut out any outside glare of light. The gemological uses of this simple instrument are limited, but it can be useful in certain circumstances.
The chromium in emerald makes it appear reddish or pinkish when the stone is strongly illuminated and viewed through the filter. Emerald imitations that do not contain chromium appear green through filter. A few natural emeralds, especially those from South America, fail to show pink through the filter. Once synthetic emeralds appeared on the market, the Chelsea filter became less useful as an emerald filter, because the synthetic product, also containing chromium, appears red through the filter, though often of a deeper red than natural counterparts.
The Chelsea filter is also useful in separating cobalt-containing gem species from others, as the presence of cobalt results in a red reaction through the filter. Gemstones colored by cobalt are in the main synthetic or man-made, and appear red through the filter. This has proven useful in separating synthetic blue spinel and man-made blue glass from the stones they imitate, namely aquamarine, blue zircon and sapphire. Gemstones containing cobalt are rare on earth.
Aquamarine and its most common simulant, synthetic blue spinela are visually very similar, but owe their respective blue color to different causes. Aquamarine is colored by iron, which absorbs red wavelengths and transmits some green wavelengths, and appears green through the filter. Synthetic blue spinel is colored by cobalt which absorbs green wavelength, but transmits red wavelength, appears red through the filter.
The absorption spectrum of aquamarine, colord by iron, and synthetic blue spinel, colored by cobalt, can help us understand why their reactions to the Chelsea filter are so different.
The Chelsea filter consists of a combination of two gelatin filters that pass wavelengths in the deep red and in the yellow green. To obtain best results, stones should be viewed under a strong artificial light with the filter held close to the eye so as to cut out any outside glare of light. The gemological uses of this simple instrument are limited, but it can be useful in certain circumstances.
The chromium in emerald makes it appear reddish or pinkish when the stone is strongly illuminated and viewed through the filter. Emerald imitations that do not contain chromium appear green through filter. A few natural emeralds, especially those from South America, fail to show pink through the filter. Once synthetic emeralds appeared on the market, the Chelsea filter became less useful as an emerald filter, because the synthetic product, also containing chromium, appears red through the filter, though often of a deeper red than natural counterparts.
The Chelsea filter is also useful in separating cobalt-containing gem species from others, as the presence of cobalt results in a red reaction through the filter. Gemstones colored by cobalt are in the main synthetic or man-made, and appear red through the filter. This has proven useful in separating synthetic blue spinel and man-made blue glass from the stones they imitate, namely aquamarine, blue zircon and sapphire. Gemstones containing cobalt are rare on earth.
Aquamarine and its most common simulant, synthetic blue spinela are visually very similar, but owe their respective blue color to different causes. Aquamarine is colored by iron, which absorbs red wavelengths and transmits some green wavelengths, and appears green through the filter. Synthetic blue spinel is colored by cobalt which absorbs green wavelength, but transmits red wavelength, appears red through the filter.
The absorption spectrum of aquamarine, colord by iron, and synthetic blue spinel, colored by cobalt, can help us understand why their reactions to the Chelsea filter are so different.
Monday, January 08, 2007
Tsavorite Garnet
National Jeweler Network writes:
Michael Coach and Associates has announced that it has acquired the largest clean tsavorite in the world. The stone weighs 325.13 carats and will be on display at the American Gem Trade Association GemFair beginning at the end of January in Tucson, Ariz.
The stone was found in the Karo area of Mereloni, Tanzania, in what is believed to be the biggest pocket of tsavorite ever discovered. The rough was found in a mine that goes straight down 120 meters from the surface and follows a reef at a 45-degree angle to a final depth of 180 meters.
Tsavorite is usually found in a host matrix of diopside, zoisite and grossular garnet, and is normally made in small nodules of gem crystal, which are put under enormous pressure. Because the nodules are squeezed and pushed during growth, inclusions are easily formed, making clean crystals something of a rarity.
From the rough, four stones were cut: a 62-carat clean, a 120-carat clean, a 307-carat included and the 325.13-carat clean acquired by Michael Couch and Associates. Asking price on the 325.13-carat stone is $2.2 million.
More info @ http://www.nationaljewelernetwork.com/njn/content_display/colored_stones/e3i359161611ed1086efe5914532f3faa3e
Michael Coach and Associates has announced that it has acquired the largest clean tsavorite in the world. The stone weighs 325.13 carats and will be on display at the American Gem Trade Association GemFair beginning at the end of January in Tucson, Ariz.
The stone was found in the Karo area of Mereloni, Tanzania, in what is believed to be the biggest pocket of tsavorite ever discovered. The rough was found in a mine that goes straight down 120 meters from the surface and follows a reef at a 45-degree angle to a final depth of 180 meters.
Tsavorite is usually found in a host matrix of diopside, zoisite and grossular garnet, and is normally made in small nodules of gem crystal, which are put under enormous pressure. Because the nodules are squeezed and pushed during growth, inclusions are easily formed, making clean crystals something of a rarity.
From the rough, four stones were cut: a 62-carat clean, a 120-carat clean, a 307-carat included and the 325.13-carat clean acquired by Michael Couch and Associates. Asking price on the 325.13-carat stone is $2.2 million.
More info @ http://www.nationaljewelernetwork.com/njn/content_display/colored_stones/e3i359161611ed1086efe5914532f3faa3e
Universities
Charles T Munger, Vice-Chairman of Berkshire Hathaway Corporation writes:
'There's a lot wrong (with American universities). I'd remove 3/4 of the faculty -- everything but the hard sciences. But nobody's going to do that, so we'll have to live with the defects. It's amazing how wrongheaded [the teaching is]. There is fatal disconnectedness. You have these squirrelly people in each department who don't see the big picture.'
'There's a lot wrong (with American universities). I'd remove 3/4 of the faculty -- everything but the hard sciences. But nobody's going to do that, so we'll have to live with the defects. It's amazing how wrongheaded [the teaching is]. There is fatal disconnectedness. You have these squirrelly people in each department who don't see the big picture.'
Burmite
Garry Platt writes:
Burma up until recent years had been China’s major source of amber. The Burmese had mined and dug for the amber which was then sold to Chinese traders who would sell on to crafts people who prized the gem for its qualities and mythical properties.
The amber was extracted from the Northern parts of the country. The mining shafts were lined with bamboo canes sunk down 10 -15 meters through clay deposits. ‘The Mineral Resources of Burma’ published in 1934 noted 200 pits aimed at extracting amber near the village of Shingban. Photographs from this period show these mines as potential death traps. The author is uninformed about the situation regarding amber mining within this region at the current time. Most records refer to activity up until the 1930’s, but none later. The age of Burmite has been established as Eocene through indicator fossils present in the amber deposits. The tree origin has been determined as Nummulites Biaritzensis.
More info @ http://www.gplatt.demon.co.uk/typesof.htm
Burma up until recent years had been China’s major source of amber. The Burmese had mined and dug for the amber which was then sold to Chinese traders who would sell on to crafts people who prized the gem for its qualities and mythical properties.
The amber was extracted from the Northern parts of the country. The mining shafts were lined with bamboo canes sunk down 10 -15 meters through clay deposits. ‘The Mineral Resources of Burma’ published in 1934 noted 200 pits aimed at extracting amber near the village of Shingban. Photographs from this period show these mines as potential death traps. The author is uninformed about the situation regarding amber mining within this region at the current time. Most records refer to activity up until the 1930’s, but none later. The age of Burmite has been established as Eocene through indicator fossils present in the amber deposits. The tree origin has been determined as Nummulites Biaritzensis.
More info @ http://www.gplatt.demon.co.uk/typesof.htm
Diamond Cut: CRISSCUT
The step cut Crisscut has 77 facets, some of which are crisscrossed, and comes in two shapes:
-Rectangular
-Octagonal.
Triangular facets are applied to the steps in the pavilion. Crisscut is the creation of Christopher Slowinski, the owner of Christopher Designs.
-Rectangular
-Octagonal.
Triangular facets are applied to the steps in the pavilion. Crisscut is the creation of Christopher Slowinski, the owner of Christopher Designs.
How To Judge Asterism
Asterism may be perceived as multiple chatoyancy.
The Star
- Is it sharp and intense?
- The star should be complete with no missing or weak rays.
- The star itself should roll or travel easily across the surface.
- The intersecting rays should meet as near as possible at the center of the stone. If the stone is cut so that one
of the arms of the star cuts across the shortest axis of the cabochon; the rays are evenly spaced and pleasing
to the eye. In the gem trade, this is described as "Open Star". If a ray is oriented across the longest axis of an
oval stone, then the rays will appear to be less pleasing to the eye. This is described as "Closer Star".
Body Color
- Does the stone have a good body color?
Clarity
- The stone should be free from cracks and fissures. Surface reaching cracks and fissures are undesirable
forming traps for dirt, weakening the stone.
Transparency
- There is a very fine balance between transparency of the stone and definition and intensity of the star. Too
much silk or inclusions will induce translucent or even opaque body color, while too little silk or inclusions
may create ill-defined star.
Cutting
- Is the cabochon shape pleasing to the eye?
- A fairly high dome is necessary to focus the star to the best advantage. This is because the dome of the
cabochon acts as a condensing lens to concentrate the rays of the star. If the dome of the cabochon is cut
as shallow curve it is liable to possess a nearly flat spot at the top producing diffused star that will almost
invisible unless the observer is directly above the stone. A well-proportioned dome with a smooth and even
curve, lacking any flat spots, should produce a sharp star visible from most angles.
- The weight below the girdle should not exceed approximately 20% of the weight of the stone. Most colored
stones are purchased by weight so excess material beneath the girdle that does not contribute to the body
color or to the star is an unnecessary expense. Some native cut stones are practically spherical. Such stones
should be valued at the weight they would be if cut to correct proportions.
- In highly transparent gemstones a concave base can be a decided advantage as it will provide greater
brilliance, although it should not contribute to the weight excessively. Ideally the back of a translucent or
near opaque stone should be flat, smooth, but unpolished. If the gem is sufficiently transparent it will show
any polishing defects such as pits or cracks on the base.
- Test the symmetry by spinning the stone on a flat surface. If the stone wobbles or the star is seen to oscillate,
this indicates an unsymmetrical stone. Also look out for any chips or nicks at the girdle.
- The surface polish should be good with no pits, nicks or pieces of matrix to disturb the movement of the star.
The Star
- Is it sharp and intense?
- The star should be complete with no missing or weak rays.
- The star itself should roll or travel easily across the surface.
- The intersecting rays should meet as near as possible at the center of the stone. If the stone is cut so that one
of the arms of the star cuts across the shortest axis of the cabochon; the rays are evenly spaced and pleasing
to the eye. In the gem trade, this is described as "Open Star". If a ray is oriented across the longest axis of an
oval stone, then the rays will appear to be less pleasing to the eye. This is described as "Closer Star".
Body Color
- Does the stone have a good body color?
Clarity
- The stone should be free from cracks and fissures. Surface reaching cracks and fissures are undesirable
forming traps for dirt, weakening the stone.
Transparency
- There is a very fine balance between transparency of the stone and definition and intensity of the star. Too
much silk or inclusions will induce translucent or even opaque body color, while too little silk or inclusions
may create ill-defined star.
Cutting
- Is the cabochon shape pleasing to the eye?
- A fairly high dome is necessary to focus the star to the best advantage. This is because the dome of the
cabochon acts as a condensing lens to concentrate the rays of the star. If the dome of the cabochon is cut
as shallow curve it is liable to possess a nearly flat spot at the top producing diffused star that will almost
invisible unless the observer is directly above the stone. A well-proportioned dome with a smooth and even
curve, lacking any flat spots, should produce a sharp star visible from most angles.
- The weight below the girdle should not exceed approximately 20% of the weight of the stone. Most colored
stones are purchased by weight so excess material beneath the girdle that does not contribute to the body
color or to the star is an unnecessary expense. Some native cut stones are practically spherical. Such stones
should be valued at the weight they would be if cut to correct proportions.
- In highly transparent gemstones a concave base can be a decided advantage as it will provide greater
brilliance, although it should not contribute to the weight excessively. Ideally the back of a translucent or
near opaque stone should be flat, smooth, but unpolished. If the gem is sufficiently transparent it will show
any polishing defects such as pits or cracks on the base.
- Test the symmetry by spinning the stone on a flat surface. If the stone wobbles or the star is seen to oscillate,
this indicates an unsymmetrical stone. Also look out for any chips or nicks at the girdle.
- The surface polish should be good with no pits, nicks or pieces of matrix to disturb the movement of the star.
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