Gem - mineral, rock, organic or synthetic material that can be collected and used for jewelry. This is a natural process and worked precious, jewelry and puonieji stones, organic gems. Minerals are often found in nature called pusbrangakmeniais. Currently, among the experts is the presumption that all the stones are known as precious stones, the line between precious and semi-precious stones disappeared. However, the simplification of these concepts, however, the stones with unusual properties have been attributed to all the usual stones. Some authors classify the rocks in the true sense of gems ( diamonds , sapphires , rubies , emeralds , spinel, topaz , alexandrites, aquamarine , heliodor, morganitai) gems in the broad sense (quartz, etc.)., puoniuosius rocks (feldspar, jade , etc..) , rare collectibles stones ( Azur , apatite , diopsidas, etc.) and synthetic gems.

Gem weight is measured in carats. Karate (called Carat, the abbreviation ct) - the measurement unit for measuring the polished jewelry and gemstone weight. In 1907. International Committee for Weights and Measures in Paris conference was a metric carat, equal to 200 milligrams (mg) or 0.2 gram (g). Before carat which the mass has changed, so the historical weight of diamonds can now cease.