Tuesday, June 2, 2009
Garnet
Introduction
Garnet is a naturally occurring gemstone.Its name comes from Latin granatus meaning seed, because it often resembles small round seeds when found in its matrix rock.Rather than a single gemstone, garnet is a family of related minerals, some of which occur as gemstones.Each has a common crystal structure, and a similar chemical composition.The popular understanding of garnet is as an inexpensive dark red stone. Because it is relatively common and inexpensive, it is often thought of as "only garnet", and as being inferior.This bias extends to other rare and attractive forms of garnet.
Colour
Garnet occurs naturally in a large range of colours including: red, orange, brown, green, yellow, and brown.
Two Families of Garnet
There are two main theoretical groups or "families" of garnet:- pyrope, almandite, spessartite, which are all (metal) aluminium silicates, anduvarovite, grossularite, andradite, which are all calcium (metal) silicates.In practice, there are probably very few garnets with the precise pure chemical composition shown for their type, almost all garnets are of mixed types, where one type is partially replaced by another type.
Trade Terminology
In the trade, gem dealers hardly ever refer to their stock as "pyrope" or "almandite". Instead, they will often refer to "brown" or Indian garnets. As we have stated, very few, if any, pure "types" of garnet occur, most specimens approximate to a particular type.Brown and purpleish "Indian" garnets will usually be towards the pyrope end of the pyrope-almandite axis.Reddish "African" garnets will usually be towards the almandite end of the pyrope-almandite axis.Rhodolite garnets are usually about midway along the pyrope-almandite axis.
Demantoid Garnet
Demantoid garnet is a rare and beautiful bright grass green sub-variety of andradite garnet. It appears to have first been discovered around 1892 in the Bobrovka area of Russia.The Bobrovka is a small tributary of the River Tschussowaja in the Sissersk region on the western side of the Ural Mountains.It was at first thought to be emerald, which is found nearby, and has been erroneously called "Uralian emerald".The name demantoid means diamond-like, because it has a very high adamantine lustre, and a colour dispersion higher than diamond. The only disadvantageous property of demantoid is its low hardness figure at about 6.5 Moh. It is the softest of the garnets, and is more suitable for use in brooches, pendants, or ear-rings, rather than rings, because of this.The brilliant colour of demantoid garnet is due to partial replacement of the silicate by chromic oxide.A diagnostic characteristic of demantoid is the inclusion of radiating fibres of byssolite (asbestos) fibres in a pattern described as a horse-tail. There is no other green stone which shows this feature.In late Victorian times, and early in the twentieth century, demantoid became a very sought after stone. It commanded high prices because it has never been available in large quantity. In recent decades, it has been unobtainable as newly mined stones, and has only been available from antique jewellery.Recently, small finds have again been made in Russia, and a small quantity of fine quality stones have recently come onto the market. Gemstone lovers wishing to acquire a piece of demantoid garnet should take this opportunity to do so. If the current seams of demantoid run out, there may be another century without new stocks of demantoid becoming available.
Tsavolite Garnet
Tsavolite, previously called tsavorite, is a bright green variety of grossular garnet, its colour being induced by the presence of chromium.
Two Families of GarnetThere are two main theoretical groups or "families" of garnet:- pyrope, almandite, spessartite, which are all (metal) aluminium silicates, anduvarovite, grossularite, andradite, which are all calcium (metal) silicates.In practice, there are probably very few garnets with the precise pure chemical composition shown for their type, almost all garnets are of mixed types, where one type is partially replaced by another type.
Trade TerminologyIn the trade, gem dealers hardly ever refer to their stock as "pyrope" or "almandite". Instead, they will often refer to "brown" or Indian garnets. As we have stated, very few, if any, pure "types" of garnet occur, most specimens approximate to a particular type.Brown and purpleish "Indian" garnets will usually be towards the pyrope end of the pyrope-almandite axis.Reddish "African" garnets will usually be towards the almandite end of the pyrope-almandite axis.Rhodolite garnets are usually about midway along the pyrope-almandite axis.
Demantoid GarnetDemantoid garnet is a rare and beautiful bright grass green sub-variety of andradite garnet. It appears to have first been discovered around 1892 in the Bobrovka area of Russia.The Bobrovka is a small tributary of the River Tschussowaja in the Sissersk region on the western side of the Ural Mountains.It was at first thought to be emerald, which is found nearby, and has been erroneously called "Uralian emerald".The name demantoid means diamond-like, because it has a very high adamantine lustre, and a colour dispersion higher than diamond. The only disadvantageous property of demantoid is its low hardness figure at about 6.5 Moh. It is the softest of the garnets, and is more suitable for use in brooches, pendants, or ear-rings, rather than rings, because of this.The brilliant colour of demantoid garnet is due to partial replacement of the silicate by chromic oxide.A diagnostic characteristic of demantoid is the inclusion of radiating fibres of byssolite (asbestos) fibres in a pattern described as a horse-tail. There is no other green stone which shows this feature.In late Victorian times, and early in the twentieth century, demantoid became a very sought after stone. It commanded high prices because it has never been available in large quantity. In recent decades, it has been unobtainable as newly mined stones, and has only been available from antique jewellery.Recently, small finds have again been made in Russia, and a small quantity of fine quality stones have recently come onto the market. Gemstone lovers wishing to acquire a piece of demantoid garnet should take this opportunity to do so. If the current seams of demantoid run out, there may be another century without new stocks of demantoid becoming available.
Tsavolite GarnetTsavolite, previously called tsavorite, is a bright green variety of grossular garnet, its colour being induced by the presence of chromium.
Topazolite
A name used for a variety of garnet with a topaz-yellow or an olive green colour.
MagnetismSome types of garnet, particularly the reddish "African" garnets, which are usually found in close association with diamonds, showing distinct magnetism.
A name used for a variety of garnet with a topaz-yellow or an olive green colour.
Magnetism
Some types of garnet, particularly the reddish "African" garnets, which are usually found in close association with diamonds, showing distinct magnetism.
Garnet is a naturally occurring gemstone.Its name comes from Latin granatus meaning seed, because it often resembles small round seeds when found in its matrix rock.Rather than a single gemstone, garnet is a family of related minerals, some of which occur as gemstones.Each has a common crystal structure, and a similar chemical composition.The popular understanding of garnet is as an inexpensive dark red stone. Because it is relatively common and inexpensive, it is often thought of as "only garnet", and as being inferior.This bias extends to other rare and attractive forms of garnet.
Colour
Garnet occurs naturally in a large range of colours including: red, orange, brown, green, yellow, and brown.
Two Families of Garnet
There are two main theoretical groups or "families" of garnet:- pyrope, almandite, spessartite, which are all (metal) aluminium silicates, anduvarovite, grossularite, andradite, which are all calcium (metal) silicates.In practice, there are probably very few garnets with the precise pure chemical composition shown for their type, almost all garnets are of mixed types, where one type is partially replaced by another type.
Trade Terminology
In the trade, gem dealers hardly ever refer to their stock as "pyrope" or "almandite". Instead, they will often refer to "brown" or Indian garnets. As we have stated, very few, if any, pure "types" of garnet occur, most specimens approximate to a particular type.Brown and purpleish "Indian" garnets will usually be towards the pyrope end of the pyrope-almandite axis.Reddish "African" garnets will usually be towards the almandite end of the pyrope-almandite axis.Rhodolite garnets are usually about midway along the pyrope-almandite axis.
Demantoid Garnet
Demantoid garnet is a rare and beautiful bright grass green sub-variety of andradite garnet. It appears to have first been discovered around 1892 in the Bobrovka area of Russia.The Bobrovka is a small tributary of the River Tschussowaja in the Sissersk region on the western side of the Ural Mountains.It was at first thought to be emerald, which is found nearby, and has been erroneously called "Uralian emerald".The name demantoid means diamond-like, because it has a very high adamantine lustre, and a colour dispersion higher than diamond. The only disadvantageous property of demantoid is its low hardness figure at about 6.5 Moh. It is the softest of the garnets, and is more suitable for use in brooches, pendants, or ear-rings, rather than rings, because of this.The brilliant colour of demantoid garnet is due to partial replacement of the silicate by chromic oxide.A diagnostic characteristic of demantoid is the inclusion of radiating fibres of byssolite (asbestos) fibres in a pattern described as a horse-tail. There is no other green stone which shows this feature.In late Victorian times, and early in the twentieth century, demantoid became a very sought after stone. It commanded high prices because it has never been available in large quantity. In recent decades, it has been unobtainable as newly mined stones, and has only been available from antique jewellery.Recently, small finds have again been made in Russia, and a small quantity of fine quality stones have recently come onto the market. Gemstone lovers wishing to acquire a piece of demantoid garnet should take this opportunity to do so. If the current seams of demantoid run out, there may be another century without new stocks of demantoid becoming available.
Tsavolite Garnet
Tsavolite, previously called tsavorite, is a bright green variety of grossular garnet, its colour being induced by the presence of chromium.
Two Families of GarnetThere are two main theoretical groups or "families" of garnet:- pyrope, almandite, spessartite, which are all (metal) aluminium silicates, anduvarovite, grossularite, andradite, which are all calcium (metal) silicates.In practice, there are probably very few garnets with the precise pure chemical composition shown for their type, almost all garnets are of mixed types, where one type is partially replaced by another type.
Trade TerminologyIn the trade, gem dealers hardly ever refer to their stock as "pyrope" or "almandite". Instead, they will often refer to "brown" or Indian garnets. As we have stated, very few, if any, pure "types" of garnet occur, most specimens approximate to a particular type.Brown and purpleish "Indian" garnets will usually be towards the pyrope end of the pyrope-almandite axis.Reddish "African" garnets will usually be towards the almandite end of the pyrope-almandite axis.Rhodolite garnets are usually about midway along the pyrope-almandite axis.
Demantoid GarnetDemantoid garnet is a rare and beautiful bright grass green sub-variety of andradite garnet. It appears to have first been discovered around 1892 in the Bobrovka area of Russia.The Bobrovka is a small tributary of the River Tschussowaja in the Sissersk region on the western side of the Ural Mountains.It was at first thought to be emerald, which is found nearby, and has been erroneously called "Uralian emerald".The name demantoid means diamond-like, because it has a very high adamantine lustre, and a colour dispersion higher than diamond. The only disadvantageous property of demantoid is its low hardness figure at about 6.5 Moh. It is the softest of the garnets, and is more suitable for use in brooches, pendants, or ear-rings, rather than rings, because of this.The brilliant colour of demantoid garnet is due to partial replacement of the silicate by chromic oxide.A diagnostic characteristic of demantoid is the inclusion of radiating fibres of byssolite (asbestos) fibres in a pattern described as a horse-tail. There is no other green stone which shows this feature.In late Victorian times, and early in the twentieth century, demantoid became a very sought after stone. It commanded high prices because it has never been available in large quantity. In recent decades, it has been unobtainable as newly mined stones, and has only been available from antique jewellery.Recently, small finds have again been made in Russia, and a small quantity of fine quality stones have recently come onto the market. Gemstone lovers wishing to acquire a piece of demantoid garnet should take this opportunity to do so. If the current seams of demantoid run out, there may be another century without new stocks of demantoid becoming available.
Tsavolite GarnetTsavolite, previously called tsavorite, is a bright green variety of grossular garnet, its colour being induced by the presence of chromium.
Topazolite
A name used for a variety of garnet with a topaz-yellow or an olive green colour.
MagnetismSome types of garnet, particularly the reddish "African" garnets, which are usually found in close association with diamonds, showing distinct magnetism.
A name used for a variety of garnet with a topaz-yellow or an olive green colour.
Magnetism
Some types of garnet, particularly the reddish "African" garnets, which are usually found in close association with diamonds, showing distinct magnetism.
Fire Opal
Fire Opal
A Variety of OpalFire opal is a variety of opal. It is bright orange in colour, and always reminds us of barley sugar sweets. It can vary from completely translucent without any opalescence or iridescence, to semi-translucent with very noticeable flashes of iridescent colour.It is often called Mexican fire opal, because Mexico is one of its prime sources.
Aztec Gold
Fire opal was known to the Aztecs between about 1200 to 1519 AD.
A Variety of OpalFire opal is a variety of opal. It is bright orange in colour, and always reminds us of barley sugar sweets. It can vary from completely translucent without any opalescence or iridescence, to semi-translucent with very noticeable flashes of iridescent colour.It is often called Mexican fire opal, because Mexico is one of its prime sources.
Aztec Gold
Fire opal was known to the Aztecs between about 1200 to 1519 AD.
Emerald
Emerald
Emerald is the grass green variety of the gemstone called beryl. Although all emeralds are beryl, not all beryls are emerald.
Colouration
Pure beryl is colourless, often called white, and although quite rare, tends not to be valuable because it does not have much brilliance. Colours, as in many gemstones, are caused by small amounts of impurity, usually metallic oxides. This is a another case where impurity is desirable.Chromium, in the form of chromic oxide, causes the bright grassy green colouring in beryl, thereby producing emeralds.Vanadium can also affect the exact shade, as may traces of iron.It is also possible to have green beryl which is not emerald, because the colouring agent is not chromium.
Hardness
Emerald, along with other beryls, is quite hard, having a hardness of 71/2 to 8 on Moh's scale, compared with 10 for diamond, 9 for corundum, and 8 for topaz. Hardness is generally a desirable feature is gemstones.
Sources
The earliest known source of emerald was near the Red Sea in Egypt, the so-called Cleopatra's emerald mines. They were probably worked from about 2000 B.C., apparently the location of them was lost in the middle ages, and not rediscovered until 1818. Most emeralds used in ancient jewellery are believed to have come from these mines. They are not worked nowadays because of the low quality of crystals found.Emeralds have been found in Austria since Roman times, in the Legbach ravine at Habachtal near Salzburg. These are no longer commercially mined.Columbia is generally recognised as the source of the world's finest quality emeralds, both in the past and the present. The Columbian Indians were using them before 1537, when Quesada conquered Columbia. Later the Spanish discovered that the emerald mines were at Somondoco, which means "god of the green stones", and which is now known as Chivor. The best coloured Columbian emeralds are said to be those from the Muzo mine, although another mine at Cosquez is also highly rated.Russia has been another important source of emeralds in the past. Most Russian emeralds coming from Sverdlovsk or Ekaterinburg in the Ural Mountains.Emeralds were discovered in Australia in 1890 in New South Wales.Emeralds were discovered between1927 and 1929 at Gravelotte in South Africa, followed by other sources.Another important source of superb quality emeralds, usually only of small size, is Sandawana in Zimbabwe formerly Southern Rhodesia. These were discovered only in 1956.Emeralds were known in India from antiquity, but their source is not certain. The earliest known Indian source was 1929 at Arawalli in Rajahstan, other sources being discovered since. The quality of Indian emeralds is very variable, but most are of lower quality which are often polished as beads.Other sources of emerald include Norway, North Carolina, Connecticut, Maine, New Hampshire, although non of these are very important.
Emerald is the grass green variety of the gemstone called beryl. Although all emeralds are beryl, not all beryls are emerald.
Colouration
Pure beryl is colourless, often called white, and although quite rare, tends not to be valuable because it does not have much brilliance. Colours, as in many gemstones, are caused by small amounts of impurity, usually metallic oxides. This is a another case where impurity is desirable.Chromium, in the form of chromic oxide, causes the bright grassy green colouring in beryl, thereby producing emeralds.Vanadium can also affect the exact shade, as may traces of iron.It is also possible to have green beryl which is not emerald, because the colouring agent is not chromium.
Hardness
Emerald, along with other beryls, is quite hard, having a hardness of 71/2 to 8 on Moh's scale, compared with 10 for diamond, 9 for corundum, and 8 for topaz. Hardness is generally a desirable feature is gemstones.
Sources
The earliest known source of emerald was near the Red Sea in Egypt, the so-called Cleopatra's emerald mines. They were probably worked from about 2000 B.C., apparently the location of them was lost in the middle ages, and not rediscovered until 1818. Most emeralds used in ancient jewellery are believed to have come from these mines. They are not worked nowadays because of the low quality of crystals found.Emeralds have been found in Austria since Roman times, in the Legbach ravine at Habachtal near Salzburg. These are no longer commercially mined.Columbia is generally recognised as the source of the world's finest quality emeralds, both in the past and the present. The Columbian Indians were using them before 1537, when Quesada conquered Columbia. Later the Spanish discovered that the emerald mines were at Somondoco, which means "god of the green stones", and which is now known as Chivor. The best coloured Columbian emeralds are said to be those from the Muzo mine, although another mine at Cosquez is also highly rated.Russia has been another important source of emeralds in the past. Most Russian emeralds coming from Sverdlovsk or Ekaterinburg in the Ural Mountains.Emeralds were discovered in Australia in 1890 in New South Wales.Emeralds were discovered between1927 and 1929 at Gravelotte in South Africa, followed by other sources.Another important source of superb quality emeralds, usually only of small size, is Sandawana in Zimbabwe formerly Southern Rhodesia. These were discovered only in 1956.Emeralds were known in India from antiquity, but their source is not certain. The earliest known Indian source was 1929 at Arawalli in Rajahstan, other sources being discovered since. The quality of Indian emeralds is very variable, but most are of lower quality which are often polished as beads.Other sources of emerald include Norway, North Carolina, Connecticut, Maine, New Hampshire, although non of these are very important.
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