There are several thousand known minerals in nature (with estimates ranging from 2,000 to 7,000), but fewer than a hundred are considered gem minerals. Of these, only about a dozen or so are actually valuable enough to be important gemstones on the world market. In order to be considered a gemstone, a mineral must first of all be beautiful. In addition, it must be hard and durable. Its value increases if it is also rare.
The beauty of a gem is measured in terms of its clarity, brilliance, and color. Its natural beauty can be enhanced by the way it is cut. There are two basic kinds of gem cuts: faceted and cabochon. The faceted cut has many flat cut surfaces (facets) with an overall shape that might be round, oval, square, rectangular, or pear-shaped. Faceted cuts are preferred for brilliant transparent stones such as diamond. The cabochon cut has a smooth rounded top, usually with a flat base, and it is mainly used for opaque or translucent stones.
Hardness is measured using the Mohs’ scale, on which 10 is hardest. (Diamond has a hardness of 10.) Gemstones should have a Mohs’ hardness of 6 or more. A really durable gem should have a hardness of at least 7, which is the hardness of quartz. Table 1 shows the hardness of some familiar minerals on the Mohs’ scale.
The value of a gemstone depends on its beauty and its rarity, but also the size of the stone. Size is measured in terms of weight using the carat as a unit. A carat is 0.2 grams (0.007 ounces). (A 10-carat diamond weighs 2 grams, or 0.07 ounces.) There are 100 points in a carat, so a 30-point diamond weighs 0.3 carat, or 0.06 grams (0.002 ounces). Since gemstones vary in density (weight per unit volume), several different 1-carat stones may vary in size, the stones with the greatest density being smaller than the others.