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.
Saturday, May 30, 2009
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
Chalcedony
Chalcedony
Chalcedony is a form of quartz which is cryptocrystalline. Microscopically small crystals of quartz are embedded in opal, which is itself hydrated quartz. The proportion of quartz to opal varies, and this variation is reflected in differing densities, refractive indexes and other properties. The quartz crystals are in a fibrous form, sometimes running approximately parallel to each other, sometimes entwined.Another name for chalcedony is agate, although the term agate is usually used to describe the banded varieties of chalcedony. Many agates are naturally coloured, others are stained for decorative effect. Varieties include chrysoprase, cornelian (carnelian, sard, sardonyx, onyx, plasma, bloodstone, myrickite. There are many differently named varieties of agate depending on their appearance such as moss agate, fossilised wood. and other forms of fossil. Jasper also belongs to this group, and contains impurities which cause heavy pigmentation.
Chalcedony is a form of quartz which is cryptocrystalline. Microscopically small crystals of quartz are embedded in opal, which is itself hydrated quartz. The proportion of quartz to opal varies, and this variation is reflected in differing densities, refractive indexes and other properties. The quartz crystals are in a fibrous form, sometimes running approximately parallel to each other, sometimes entwined.Another name for chalcedony is agate, although the term agate is usually used to describe the banded varieties of chalcedony. Many agates are naturally coloured, others are stained for decorative effect. Varieties include chrysoprase, cornelian (carnelian, sard, sardonyx, onyx, plasma, bloodstone, myrickite. There are many differently named varieties of agate depending on their appearance such as moss agate, fossilised wood. and other forms of fossil. Jasper also belongs to this group, and contains impurities which cause heavy pigmentation.
Cat's Eye
Cat's Eye
True cat's eye is a variety of chrysoberyl, sometimes called cymophane, and as such is related to alexandrite.It is a rare and valuable gemstone, which is often underrated as being a "semi-precious" stone.
Chatoyancy
Chatoyancy or chatoyance, literally from French means cat's eye. It is the effect seen best in this variety of chrysoberyl, but also found in a few other gemstones. The effect is a strongly banded one caused by tube-like or needle-like crystals running along the length of the stone.Other stones which show chatoyancy include sapphire and tourmaline.Chatoyancy is related to asterism.
Cabochons
Cat's eye chrysoberyls are always cut as cabochons to show off their spectacular effect to it best.
Colour
Cat's eye chrysoberyl is green or yellow-green, yellow, or brown to yellow-brown
True cat's eye is a variety of chrysoberyl, sometimes called cymophane, and as such is related to alexandrite.It is a rare and valuable gemstone, which is often underrated as being a "semi-precious" stone.
Chatoyancy
Chatoyancy or chatoyance, literally from French means cat's eye. It is the effect seen best in this variety of chrysoberyl, but also found in a few other gemstones. The effect is a strongly banded one caused by tube-like or needle-like crystals running along the length of the stone.Other stones which show chatoyancy include sapphire and tourmaline.Chatoyancy is related to asterism.
Cabochons
Cat's eye chrysoberyls are always cut as cabochons to show off their spectacular effect to it best.
Colour
Cat's eye chrysoberyl is green or yellow-green, yellow, or brown to yellow-brown
Cornelian
Cornelian
Cornelian is a red variety of chalcedony which is cryptocrystalline quartz. Its red colour is due to the presence of iron impurities in the form of iron oxide or hematite. It can vary from a flesh red to a clear red. It is usually cut en cabochon, or into beads, and is also used for intaglios and cameos. Imitations of cornelian are made by the staining of agate.
Sources
Originally found in the deserts of Arabia and Egypt, it has also been found in the Rio Grande area, India, China, Colombia, Saxony, Scotland, West Germany and USA.
Cornelian is a red variety of chalcedony which is cryptocrystalline quartz. Its red colour is due to the presence of iron impurities in the form of iron oxide or hematite. It can vary from a flesh red to a clear red. It is usually cut en cabochon, or into beads, and is also used for intaglios and cameos. Imitations of cornelian are made by the staining of agate.
Sources
Originally found in the deserts of Arabia and Egypt, it has also been found in the Rio Grande area, India, China, Colombia, Saxony, Scotland, West Germany and USA.
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