Of the some 2000 minerals that have been identified, only about 90 have varieties that produce specimens possessing the requisite beauty and durability to be considered gemstones. Of this 90, only about 20 are particularly important to the jeweler.
Since most gemstones are minerals, the classification method used in gemology is the same one applied by mineralogists to the various minerals, with minor adjustments. Each mineral that produces gemstones is considered a gem SPECIES, A gem species is characterized by a definite chemical composition and usually a characteristic crystal structure. Therefore, each species possesses its own characteristic properties, However, most species include, a number of different types of material with variations that are usually based on color and transparency; each of these is called a VARIETY.
For example, ruby and sapphire are both varieties of the gem species corundum. Ruby is the red variety and sapphire is the name applied to blue and all other colors. Since both are corundum, ruby and sapphire have the same chemical composition, the same crystal structure, and the same properties. They differ only in color. By the same token, emerald and aquamarine are varieties of the gem species beryl. There is one other classification of importance in gem materials: the GROUP. A group is a number of closely related species. There are two groups of importance in the study of colored stones. The first is garnet. It is composed of a number of minerals having the same crystal structure but with variations in chemical composition. Thus the garnet group has a number of closely related species. The second is the feldspar group, which includes amazonite and moonstone as varieties of different species.
The jades, the two important species of which are jadeite and nephrite, are not members of the same group, although they are often treated as a group by gemologists because of their similar characteristics. The more important gem materials and their principal varieties are listed on the accompanying table.