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發表於 2010-8-22 10:57 PM
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Cat's Eye Gemstone | Chatoyant Chrysoberyl | Catseye
History and General Information- Chrysoberyl Cat's Eye
The translucent honey-yellow to greenish or brownish yellow chatoyant chrysoberyl which, when cut with a domed surface (cabochon) show a mobile streak of light are called as cymophane or cat's eye chrysoberyl. cat’s eye is a band of light that runs length wise from one end of the stone to the other. This effect is called chatoyancy.
In gemology, chatoyancy (or chatoyance) is an optical reflectance effect seen in certain natural gemstones. Coined from the French "oeil de chat", meaning 'cat's eye', chatoyancy arises either from the fibrous structure of a material, as seen in tiger eye quartz, or from fibrous inclusions or cavities within the stone, as seen in chrysoberyl cat's eye .
This chatoyant chrysoberyl Catseye gemstone is also known as ‘cymophane’ which comes from a Greek word that means ‘wavelike form’, owing the name to the chatoyancy sometimes exhibited in this variety of gemstone.
Cat’s eye stones (gemstones displaying the streak of light), similar in appearance to cymophane, are formed in minerals such as tourmaline, scapolite, corundum, spinel and quartz. The industry designates these gemstones as quartz cat's eyes, or ruby cat's eyes etc and only the chrysoberyl gemstone can be referred to as cat's eye with no other designation.
This gem has had the longest history amongst the other chrysoberyl varieties. By the start of the nineteenth century the gem had lost its popularity. However, in the late nineteenth century when the Duke of Connaught gave a cat's eye ring as an engagement token to the princess of Prussia, this gemstone became popular once again and its value increased greatly.
In India, this gem is known by various names like ‘lasunia’, ‘vaidhuriya’, ‘ketu’, and ‘bidalaskh’. The gemstone is one of the Navagraha stones (stones that represent the nine planets that have a cosmic influence on all earthlings) representing the God of the Descending/South filter_194r node (ketu) in Indian astrology and is related to supernatural influences.
This gem is believed to protect the wearer from bad fortune, harmonizing the negative effects of Ketu and hence it is popular in the Indian market. For therapeutic purposes, chrysoberyl rings should be worn as recommended. Chrysoberyl stones are tested and cut for the specific purpose and can be set in a ring in a specific format.
The cat’s eye gemstones and cats eye beads are mostly used in jewelry item like rings, earrings, bracelets, necklaces and more.
Composition of Chrysoberyl Cat's Eye
The chemical formula of this gem is beryllium aluminum oxide (BeO Al2 O3). The specific gravity ranges between is 3.72 and 3.74 and the refractive index range is between 1.746 and 1.755 (Spot 1.75). Its hardness on Mohs scale is 8.5.
Varieties
The chrysoberyl cat’s eye belongs to the Chrysoberyl group. The 2 main varieties of the Chrysoberyl group are:
1. Alexandrite Gemstone (color changing)
2. Cymophane (chatoyant Cat’s eye)
Physical Properties of Chrysoberyl Cat's Eye
Cats eye Color
The quality of this gem is judged by the display of chatoyancy effect. A cat’s-eye ( Chatoyancy) is the band of light that runs length wise from one end of the stone to the other. Chatoyancy is caused by a parallel needle or a hollow tube like inclusion inside the gem. When the gem is rotated while light is flashed on the gem, it gives an image of the opening and closing of an eye, thus its name.
It is found in a range of colors from yellow to yellowish green, greyish green to brown and brownish yellow.
Cut
The natural chrysoberyl rough must be cut in a manner that highlights the cat’s eye effect. This means that the natural chrysoberyl gem should display the opening and closing of the cat’s eye when light is thrown on it and the gem is rotated.
The needles in the stone must be perpendicular to the stone. Thus, a very skillful and experienced lapidarist is required to cut this gem. The gem should be cut in a dome-like oval or round shape, also know as “cabochon cut” ( not faceted, just plain polished dome ) and polished to display its chatoyancy.
Carat
The commonly used size of the gem in the market is between 0.10ct and 3.00cts. However gems up to 150 carats in weight are also known to exist. Large gems are rare and are of higher value.
Clarity
The gem by nature has a milky appearance and transparent gems are almost never found.
Pricing and Buying of Chrysoberyl Cat's eye
The cymophane or the ceylon cats eye is the most valuable and expensive gem of all the cat’s eyes gemstones in the market. The chatoyancy quality of, the cut of the gem and the color are the main factors that decide the prices that they can fetch in the market.
A stone with a distinct, straight and silver-white well-centered band would be considered to be of the best quality and fetches the best price. A semi-transparent honey color, with the honey color on one side of the band and milky appearance on the other (when light falls on the stone) is considered the most desirable color. In the market this gem can range anywhere from $15 to $2500 per carat.
Markets and Producers
The finest gemstones that are used in cats eye jewelry are mined in Sri Lanka, while fine and good quality gem are also found in India, Brazil, Burma and Madagascar. The gem is in high demand in Japan and Hong Kong for use in finger rings.
Treatments done on Cat's Eye
To enhance the color of a $200 per carat stone to a more desirable honey-brown worth $1,500 per carat or more, the stones are bombarded in an atomic reactor. The radiation is more than 50 times greater than acceptable U.S. limits. Exposure to such high levels of radiation could cause all kinds of health problems, including skin-cancer and therefore carrying out the treatment is dangerous.
Hundreds of such highly radioactive chrysoberyl cat's eye gemstones are sold in some filter-074 markets where the stone is in great demand. These have been in circulation in Indonesia, Bangkok, Hong Kong and Japan since April of 1997, according to some industry sources.
Simulants of Chrysoberyl Cat's Eye
Most of the other gemstones that display chatoyancy, like the quartz cat’s eye, fibrolite, and apatite and tourmaline cat’s eye could be used as simulants of the chrysoberyl cat’s eye.
Interesting Trivia and other facts
Cat's eye chrysoberyl and the color changing alexandrite gemstone belong to the same family. They can be counted among the most remarkable gems ever known. Among the alexandrite gemstones a few have been found which combines the characteristics of the cat's eye and the alexandrite and were thus named as the alexandrite cat's eye.
Astronomically, Ketu and Rahu denote the points of intersection of the paths of the Sun and the Moon as they move on the celestial sphere. Therefore, Rahu and Ketu are respectively called the north and the south filter_194r nodes. Ketu is generally referred to as a ‘shadow’, and is considered as ‘Tail of the Demon Snake’. It is believed to have a tremendous impact on human lives and also the whole creation.
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