To the average person it might seem that a jeweler’s showcase of gems presents innumerable kinds of precious stones, when actually only a few species of minerals are there. Perhaps only diamond, ruby, emerald, aquamarine, sapphire, opal, tourmaline, and amethyst would comprise the entire stock. Yet, since the mineral kingdom consists of about 2000 distinct species, it would seem that a few more kinds of gemstones would be available. Certainly, many more minerals than are seen displayed by the jeweler have been used as gems over the centuries. The study of all these species of gem minerals constitutes modern gemology—a specialized branch of the science of mineralogy. With the few exceptions already noted, all gems are minerals found in the earth’s crust. A mineral is a natural substance having a definite chemical composition and definite physical characteristics by which it can be recognized. However, for a mineral to qualify as a gem it must have at least some of the accepted requirements—brilliance, beauty, durability, rarity, and portability. Of course, if a gemstone happens to be “fashionable” it will have additional importance. Rarely does a single gem possess all of these qualities. A fine-quality diamond, having a high degree of brilliance and fire, together with extreme hardness and great rarity, comes closest to this ideal, and in the world of fashion the diamond is unchallenged among gems. The opal, by contrast, is relatively fragile, and it depends mainly on its rarity and its beautiful play of colors to be considered gem material. When a gem material, as found in nature, has at least a minimum number of the necessary qualities, it is then the task of the lapidary, or gem cutter, to cut it and polish it in such a way as to take greatest advantage of all its possibilities for beauty and adornment. PHYSICAL CHARACTERISTICS OF GEMSTONES When a gemologist or a gem cutter examines an unworked mineral fragment (called _rough_) he looks for certain distinguishing characteristics that will aid him in identifying the mineral and in determining the procedures he should use in cutting it. Scale of Hardness Soft 1. Talc ^ 2. Gypsum 3. Calcite 4. Fluorite 5. Apatite 6. Feldspar 7. Quartz 8. Topaz v 9. Corundum Hard 10. Diamond