The opaque gems include representatives of metallic ores and rock-forming minerals, and with the exception of jade they are used mainly as ornamental stones. Some opaque gems such as obsidian, a volcanic glass, and jet, ...

The transparent gems are known for their striking luster and brilliance along with their color and hardness. Most transparent gems are oxides of aluminum, beryllium, or magnesium, with a few containing silica. Some transparent gems ...

For more than 6,000 years, people have cherished and fought over gold, and this metal was responsible in part for the making of civilization. Gold is the universally accepted medium of exchange. It is also ...