jueves, 15 de mayo de 2008
No one knows how long ago man developed a love for gems. Every civilization unearthed by archeologists show that its members prized certain colorful minerals for personal ornamentation or sometimes, as in de case of jade of the Chinese, held some as a sacred. The cutter and carver of gem materials enjoyed special privileges because only he knew the secrets of stones entrusted to his care and, seemingly by magic, could shape them to his will despite the fact that they were supremely harder than the finest metals. The ancient Greeks, and after them the Latin's, gave to us lapidary, from lapis, or stone, as the name of the artisan who carves, shapes, or engraves precious stones or gems. For hundreds of years the art of the lapidary was a secretive one and only in the last few decades has it become known, to the point where anyone at all can learn how to fashion the raw material –gemstones- into gems, carving, and many other objects of ornament.
Before we go further, it is a good idea to define gemstone, since the term does not always refer to a mineral. Originally, most raw materials used for gems came from the rocks and minerals in the earth’s crust, but in a few favored places in the world, primitive man discovered beautiful pearls within oysters and, in certain sedimentary formations, amber and jet; and because of their beauty and value, they too were classed as gemstone. Later, such materials as tortoise shell, pearl shell, coral, and a number of types of ivory were added to the list, and in most modern books on gemology, these are described and discussed along with mineral gemstones.
About 150 years old, Arthur Aikin, Secretary to he Geological Society of Great Britain, wrote a small mineralogy is which he listed about 250 minerals that were well recognized at the time.
By the close of the nineteenth century, the list had grown to about 750 species, today it is over 2000. Despite the rapid rise in the number of minerals recognized as separate species, there are still only about a hundred that provide gems from suitable pieces of rough, of these, only about twenty-five are known to the layman.
The mineral species furnishing the principal gemstone of greatest value, or the classical precious stones, are:
Corundum (ruby and sapphire)
Beryl (esmerald, aquamarine, etc.)
Crysoberyl (alexandrite, catseye, etc.)
Opal (White and black)
The following species furnish important gemstones, or those regularly handled in the gem and jewelry trade:
Feldspar group (moonstone, amazonite, etc.)
Quartz (amethyst, citrine, chalcedony, etc.)
At this point it is wise to distinguish between certain gemstone that are compose of several minerals and, hence, are ore like rocks, geologically speaking, than single mineral species. Prominent among such are marbles, which though usually composed of a single species, in this instance, calcite, occur in such quantity that they are classed as rocks rather than as large masses of as single mineral. Other rocklike gemstone are serpentine, lapis lazuli, various granites, and other stones that may or may not be good enough to use in jewelry but are frequently carved or cut into ornamental objects, often of large size. The following list of gemstones of lesser importance includes both mineral species and rock types, the latter designated by the letter R:
The following gemstones are so rare, or for some reason unsuited for use in jewelry, that they are seldom seen outside collections:
THE QUALITIES OF GEMSTONES
What qualities distinguish gemstones from ordinary rocks and minerals? Why are certain of them prized and others scorned? There is no doubt that the most important quality is beauty, for without it, a mineral or rock will not be prized, regardless of what else it has. Gemstones are therefore attractive to the eye, primarily because of vivid coloration, often enhanced by clever cutting, but also because of intriguing patterns, inclusions, or some other characteristic that lifts them above the ordinary.
Beauty is evident in the strong coloration of red ruby, blue sapphire, and green emerald, but it is also visible in the patterns of agate, in the spangles of aventurine, and in the gleaming coppery needles of rutilated quartz.
Since gems are meant to be worn, they are liable to be rubbed, chipped, or knocked against hard objects. It is important that they resist this quality soon proves it self in gems set in rings or bracelets. Those that are strong and hard, like diamond, ruby, sapphire, and chrysoberyl, will sparkle and gleam with undiminished luster after years of wear have eroded the gold or platinum of the settings to the point where stones may be lost unless the settings are repaired. On the other hand, durability is not all-important.
Consider opal as an example: this gemstone is brittle and soft, and, when worn in rings, soon