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Polishing the stones

Posted September 24th, 2008 by admin

For polishing agate, lapis lazuli, turquoise, and malachite, beechwood rollers faced with soft leather are often used. These revolve at about 200 to 400 revolutions per minute. Chinese jade is polished with silicon carbide wheels, and lubricated with weak vinegar. Opal is best polished with leather discs coated with tripoli, which is really a finely grained silica, and obtained chiefly from the U.S.A. Rotten stone is a residual product derived from the weathering of siliceous clayey limestone. It is soft, friable, grayish-brown in color, with a fine, earthy texture. In chemical composition, it varies, but it is largely composed of silica, with some alumina, carbon, iron oxide, and lime.

For very many years up to 1939, the great majority of gem stones other than the precious varieties was cut in Germany. During the war years, few stones for jewelry were fashioned as all efforts were concentrated on producing material useful for war Industries. But a gradual resumption to more normal conditions has taken place, although various financial restrictions between  some countries and the lack of rough material has made progress slow.