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Jewelry News09/05/2008 09/05/2008 Refer a FriendStore Information |
Quartz Crystal
 SILICA gems, based on quartz, contain the two most common chemical elements in the earth's crust in combination: the gas oxygen and the non-metallic element silicon, somewhat akin to carbon in its properties. It is not, therefore, surprising that this family also forms the largest group of gemstones, both in the number of varieties and in the quantities mined. It includes one truly precious species, opal, and a large number of less precious stones, down to the common varieties of jasper. For gemological purposes it is usual to classify the family into varieties which provide transparent or sub-transparent gems cut from a single crystal, collectively reffered to as 'quartz'; translucent crypto-crystalline varieties covered by the term 'chalcedony', if mainly pure, or 'jasper', if opaque and mixed with other minerals; and 'opal' and 'natural glass' for the amorphous varieties. The name 'quartz' derives from the German Queretz meaning 'crossing ore' from its way of running across other mineral veins. When pure, it is absolutely colorless and is called rock crystal. Small allochromatic admixtures of coloring agents give it a wide range of colors. The colored quarts are more highly esteemed as gems; rock crystal is now rarely cut and set as jewellery. One delightful form of art was to engrave it from the back of a flat cabochon and then to color the engraved design. Some minute pictures of great delicacy were created in this way as well as a great range of more homely art. Well-made and artistically designed rock crystals jewellery of the seventeenth and eighteenth centuries can be seen in many jewel treasuries. Quartz mainly occurs as an essential component of igneous rocks. It percolates into any cracks and lines any cavities that exist; can then form euthedral crystals. About 30 years ago a climber found some well-formed rock crystals in a remote part of the Alps. They were several feet high and weighed from 200 to 1300 pounds. The Austrian army mobilized a helicopter and some troops to bring them safely down to a museum in Salzburg.
Rock crystal is now synthesized extensively for industrial and scientific use. Synthetic quartz, tined in various colors, has been cut as gemstones in Russia. Of the colored varieties of quartz amethyst is pre-eminent. Until the eighteenth century it ranked among the most precious stones, an emblem of high rank and wealth, but its value fell with the discovery of a large South American deposit in about 1760. Even so, no other stone can match the rich deep purple of a well-colored amethyst. Stones that show a distinct tendency towards a reddish-purple are preferred. A-methystos means 'not tipsy' in Greek and the name was given to the stone in belief that it would preserve the wearer from drunkeness. The belief derives from a charming ancient legend about a most abstemious nymph; it was not always made clear that the protection from drunkeness only extended to those who eschewed alcoholic liquor! The purple color is thought to be caused by finely dispersed iron. Heat treatment changes this color to brown or yellow, according to its intensity, and many citrines are in fact heat treated amethysts. While amethyst and naturally colored citrine display distinct dichroism, 'heated' citrine doesn't. Amethysts of the finest color come from Uruguay and the neighbouring part of Brazil. Fine stones are also found in the Ural mountains (Russia) and in Japan, but stones of lesser color have a fairly wide distribution.  Another semi-transparent to translucent variety is rose quartz, colored a subtle shade of pink by manganese. This is rather rarer than the varieties so far named. It comes from Minas Gerais, Brazil and from South-West Africa, sometimes in pieces large enough to be carved into quite large figurines. It seldom occurs in well-shaped crystals. It is famous for producing translucent six-rayed star stones, the star being particularly bright in transmitted light (light which has passed through the stone). For this reason quartz stars are usually polished on their base as distinct from star corundums which are left rough. The base than acts as a mirror reflecting the incident light and enhancing the star. Sometimes colored varnishes are applied to the base to produce star stones of many colors. These can be distinguished from star corundums in that a reflection of the light source can be seen at the centre of the star. At some sites quartz has crystallized round pre-existing minerals, completely enclosing and sometimes altering them. This process has produced some striking effects. Quartz enclosing the delicate fibers of amianthus, a kind of asbestos, gives rise to cat's-eyes that may approach cymophane in quality. Quartz cat's-eyes may be yellow, brown, green or even black. They are found in the Ceylon gem gravels, like cymophane, and also in Bavaria. The coarser, wavy fibers of crocidolite asbestos are enclosed in tiger's eye. This is a golden-brown variety often cut as brooch stones or larger ornamental objects since it occurs in substantial layers. 
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