According to the Smithsonian Rock and Gem, there are 4,000 identified minerals, with fewer than 100 used as gemstones, and a minority of those gemstones that are of major significance. The same source defines a gemstone as "any mineral that is highly prized for its beauty, durability, and rarity." Gemstones themselves are divided into two major categories: Precious and semiprecious. With few exceptions, which gemstones are beautiful, durable, and rare enough to be considered precious or semiprecious has changed, and will continue to change over time.

History
 
 
Historically, there are four gemstones considered without question to be precious. Those are diamonds, rubies, sapphires, and emeralds. Pearls have on occasion made the list, though they are technically not stones or minerals. In the past, opals have also been considered precious gems. There is no one definition for or list of semiprecious stones because opinions vary with time, with culture, and with fashion trends.

Beauty
Most people know that valuation of a diamond depends upon the four C's: Cut, color, clarity, and carat weight. This universal system, created by the Gemological Institute of America, was developed for evaluating colorless diamonds. However, its principles can be applied to other gemstones. The weight given to each factor depends on the gemstone to which it is applied. For example, a diamond is valued more highly if it has fewer inclusions and, thus, higher clarity, but a rutilated quartz is valued for its inclusions.

Durability
Mohs scale of mineral hardness can be used to determine the durability of gemstones. Diamonds are the hardest known mineral and rank 10 on the Mohs scale. Rubies and sapphires, both forms of corundum, rank next highest at 9. Emeralds are next in line at 7.5 to 8 on the Mohs scale. Certainly, it is no coincidence that these four precious gemstones are among the hardest and most durable known to man.

Rarity

In ancient Greece, amethyst was considered a precious stone. It was highly prized by many who believed it could combat intoxication. Following the discovery of a large amount of amethyst in Brazil, the stone lost value because it became abundant in supply. Though it is a beautiful stone and very durable at 7 on the Mohs scale, most amethyst is no longer rare enough to be considered precious.

Terminology
Arguably, gemstones that are beautiful, durable, and rare are precious while those gemstones lacking one or more of these qualities is merely semi-precious. Unfortunately, this simplistic terminology has become misleading. Those in the gem trade no longer focus on the distinction between precious and semi-precious. All kinds of stones are used in expensive jewelry and in other fashion trends. The only definitive determination of how precious a gemstone is is the value its owner places upon it.