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.
Sunday, May 31, 2009
Diamond
Diamond is Carbon
It may seem surprising that diamond is simply carbon, just like charcoal or graphite. In fact carbon has at least two other rare, and only recently discovered forms, or allotropes, known as fullerenes.
The difference is caused by the different types of bonding between adjacent atoms to form different types of crystalline structure.
In diamond, each carbon atom is bonded to four other carbon atoms in a tetrahedral structure, like a pyramid. Each link or bond is the same length, and the tetrahedral formation is therefore completely regular. It is the strength and regularity of this bonding which makes diamond very hard, non-volatile and resistant to chemical attack.
Theoretically a perfect diamond crystal could be composed of one giant molecule of carbon.
Carbon is a non-metallic element with the atomic number of 6, and an atomic Weight of 12. In combination with oxygen and hydrogen it is contained by all living objects. In the form of graphite it appears black or dark gray, opaque, and is very soft, whereas in the form of diamond is it clear, colourless, and extremely hard. In fact diamond is the hardest known naturally occurring substance.
Carbon has a density of about 2.3 and diamond about 3.5.
What Makes Diamond An Ideal Gemstone?
Diamond possesses many qualities which make it an ideal gemstone.
It is extremely hard, and also very tough and hard-wearing, and this also helps it to take a very high polish. Some hard articles are brittle which detracts from their durability. There are some things which are harder than diamond.
In its pure form it is colourless, has a high refractive index, so has a very high lustre.
It possesses high dispersion, meaning that different light wavelengths are diffracted differently, giving a strong scintillating play of prismatic colours.
Diamond Discoveries
Diamonds seem to have been known for about 3,000 years, being mentioned in Exodus chapter 28, however in early times, other hard minerals were often confused with diamond.
It is thought that the earliest diamonds were found in about the 12th century B.C., in India , which remained the most important, if not the sole, source until 1725, when diamonds were discovered in Brazil.
The Indian and Brazilian deposits had been almost exhausted when in 1866, the Eureka diamond was discovered in South Africa, followed by the Star of South Africa in 1869. Shortly afterwards, the great South African diamond rush had started, and South Africa remains one of the world's most important sources of diamonds today.
Diamonds have since been discovered in many other regions of the world, including Russia and Australia.
Until the South Africa finds, diamonds were so rare and valuable, that they were only owned by the very wealthy. They were not available in high street shops!
Why Are Diamonds So Popular?
Through the publicity and promotion given to diamonds largely by the De Beers Company, and through the Diamond Promotion Service, diamonds have become the most desired gemstone.
Thanks to large scale mining, and the development of efficient cutting methods and equipment, diamonds have now become a consumer luxury affordable to the masses.
Mass production jewellery manufacturing techniques have also helped to bring diamond rings and other diamond jewellery into very affordable, even commodity, price ranges.
Man Made
Another fact about diamond which surprises most people, is that more diamond is now manufactured than mined. Synthetically produced diamonds have been made since at least 1954, although the bulk of the synthetic production is used for industrial purposes as diamond grit.
Gem quality synthetic diamonds have also been produced, although it is still more expensive to manufacture them than to mine them.
Pure diamond, if such a thing exists, is colourless.
Most diamonds are slightly coloured, even if the colouring is almost imperceptible. The commonest colour is yellow which is caused by tiny amounts of nitrogen being present in the crystal structure, others are gray, light brown or greenish. Diamond can be almost any colour, although strongly coloured attractive specimens are very rare, and as such are not actively promoted by De Beers, probably in case consumers realise that blue, red, green and other colours are attractive, and start buying sapphires, rubies, emeralds and other gemstones instead of diamonds!
As with many gemstones, colours can be artificially produced or modified. Some fancy coloured diamonds are produced by irradiation and subsequent heat treatment. Natural fancy coloured diamonds command very high prices, especially the more more popular colours. Fancy coloured diamonds where the colour has been artificially produced are no less beautiful, but sell for more normal prices.
We occasionally have fancy coloured diamonds available in blue, green, yellow, orange, pink, purple, brown, and even black.
It is well known that most diamonds contain slight imperfections or flaws, indeed I firmly believe that there is no such thing as a perfect diamond. The higher the clarity grade of a diamond, the higher its desirability, and therefore its price.
Most jewellers will tell you that "cut" is an important factor in the price of a diamond. While this is true, most do not know clearly what they mean by the word "cut". It can have several different meanings.
Firstly, it can describe the shape and facetting pattern of the diamond, as in the modern round brilliant cut, the single or eight cut, pear shape, emerald cut, square, baguette, oval, heart, triangle, princess, marquise or fancy.
Secondly, it can mean the accuracy of the facetting, and the proportions of the stone, and lastly it can apply to the polish or surface finish of the stone.
Saturday, May 30, 2009
Demantoid Garnet
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.
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.
Corundum
Corundum
Corundum is the mineralogical or gemmological name for aluminium oxide.It is better known for the two closely related varieties of gemstone, sapphire and ruby.
ColoursCorundum can be colourless, red, pink, blue, black, brown, orange, yellow, green, indigo, violet, or mauve. Red corundum and most pink corundum is called ruby, blue corundum is called sapphire, and other colours are also called sapphire, usually with the colour specified as a prefix to the word sapphire, for example, yellow sapphire.
Colouration
Pure corundum 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 case where impurity is desirable.Chromic oxide causes brilliant red colouring in corundum, thereby producing rubies.Ferric oxide causes yellow colouration, titanium oxide produces vivid blue.In fact the colouration of corundum is not quite so simple as this. The titanium and iron are usually present in the form of ilmenite, a mineral which is a titanium iron oxide, TiFeO3. Ilmenite is not isomorphous with aluminium oxide. Isomorphous means being able to replace the host mineral within its crystal structure. Instead ilmenite is present as a microscopic inclusion, in the form of colloidal particles.This colloidal nature may be responsible for other optical effects such as "silk", asterism, and colour banding.
Corundum is the mineralogical or gemmological name for aluminium oxide.It is better known for the two closely related varieties of gemstone, sapphire and ruby.
ColoursCorundum can be colourless, red, pink, blue, black, brown, orange, yellow, green, indigo, violet, or mauve. Red corundum and most pink corundum is called ruby, blue corundum is called sapphire, and other colours are also called sapphire, usually with the colour specified as a prefix to the word sapphire, for example, yellow sapphire.
Colouration
Pure corundum 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 case where impurity is desirable.Chromic oxide causes brilliant red colouring in corundum, thereby producing rubies.Ferric oxide causes yellow colouration, titanium oxide produces vivid blue.In fact the colouration of corundum is not quite so simple as this. The titanium and iron are usually present in the form of ilmenite, a mineral which is a titanium iron oxide, TiFeO3. Ilmenite is not isomorphous with aluminium oxide. Isomorphous means being able to replace the host mineral within its crystal structure. Instead ilmenite is present as a microscopic inclusion, in the form of colloidal particles.This colloidal nature may be responsible for other optical effects such as "silk", asterism, and colour banding.
Coral
Organic Gems
Coral is an organic gemstones. Most gemstones are formed from inorganic substances, but a number of gemstones are from organic sources, that is from living things, either plants or animals. Coral is the "skeleton" formed by the individual polyps, plant-like animals, which make up a colony.
Colours
Red and pink are the usual colours for coral, but white and black are also known.
Forms
Coral is usually polished en cabochon or into beads, but is also often carved.
Coral is an organic gemstones. Most gemstones are formed from inorganic substances, but a number of gemstones are from organic sources, that is from living things, either plants or animals. Coral is the "skeleton" formed by the individual polyps, plant-like animals, which make up a colony.
Colours
Red and pink are the usual colours for coral, but white and black are also known.
Forms
Coral is usually polished en cabochon or into beads, but is also often carved.
Citrine
Citrine
Citrine is the clear yellow or golden form of the mineral quartz. As such it is related to amethyst which is the purple form of quartz, and also to rock crystal which is the colourless variety.
Dual Personality
It is possible for some specimens of quartz to be different colours in different areas. Amethyst and citrine are varieties of quart which can both occur in the same stone. As our photograph clearly shows, the difference in colour at the two ends create a striking contrast. Such stones are sometimes called ametrines, but we think they could equally well be called citrysts or citrethysts.
Hardness
The hardness of citrine is 7, and appears on the Moh scale as quartz.
Citrine is the clear yellow or golden form of the mineral quartz. As such it is related to amethyst which is the purple form of quartz, and also to rock crystal which is the colourless variety.
Dual Personality
It is possible for some specimens of quartz to be different colours in different areas. Amethyst and citrine are varieties of quart which can both occur in the same stone. As our photograph clearly shows, the difference in colour at the two ends create a striking contrast. Such stones are sometimes called ametrines, but we think they could equally well be called citrysts or citrethysts.
Hardness
The hardness of citrine is 7, and appears on the Moh scale as quartz.
Chrysoprase
Chrysoprase
Chrysoprase is a green variety of chalcedony which is cryptocrystalline quartz. Its green colour is due to the presence of nickel impurities in the form of oxides or silicates of nickel. It can vary from a greenish yellow to a clear apple green. It is usually cut en cabochon, or into beads, and is also used for intaglios and cameos. Imitations of chrysoprase are made by the staining of agate.
Sources
Chrysoprase was known from ancient Greek and Roman times, although its ancient source is not known. It has been found at Zabkowice in Silesia, the Urals, California, and other parts of the USA.
Chrysoprase is a green variety of chalcedony which is cryptocrystalline quartz. Its green colour is due to the presence of nickel impurities in the form of oxides or silicates of nickel. It can vary from a greenish yellow to a clear apple green. It is usually cut en cabochon, or into beads, and is also used for intaglios and cameos. Imitations of chrysoprase are made by the staining of agate.
Sources
Chrysoprase was known from ancient Greek and Roman times, although its ancient source is not known. It has been found at Zabkowice in Silesia, the Urals, California, and other parts of the USA.
Chrysoberyl
Chrysoberyl
Chrysoberyl is not one of the best known gemstones. Its rarest variety alexandrite, however, is quite well known, although the number of people who have heard of alexandrite is probably 100 times greater than the number who have ever seen one, and 1,000 times greater than the number who have ever owned one.Chrysoberyl is composed of Beryllium Aluminium Oxide, is harder than topaz, and includes a variety known as cat's eye, in addition to alexandrite which we have already mentioned.
Colour
Most chrysoberyl is green or yellow-green, but some is brown. The rare alexandrite shows a colour change from green to red, and cat's eye has a strongly banded appearance which is well described by its name, and is also usually green or yellow-green.
Chrysoberyl is not one of the best known gemstones. Its rarest variety alexandrite, however, is quite well known, although the number of people who have heard of alexandrite is probably 100 times greater than the number who have ever seen one, and 1,000 times greater than the number who have ever owned one.Chrysoberyl is composed of Beryllium Aluminium Oxide, is harder than topaz, and includes a variety known as cat's eye, in addition to alexandrite which we have already mentioned.
Colour
Most chrysoberyl is green or yellow-green, but some is brown. The rare alexandrite shows a colour change from green to red, and cat's eye has a strongly banded appearance which is well described by its name, and is also usually green or yellow-green.
Friday, May 29, 2009
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