Precious gems are comprised of the traditional four precious stones which are diamonds, rubies, sapphires and emeralds. There is not a system to order the four precious gems because their beauty is a matter of taste. Tastes change with time and what is considered a favorite today may be different a few years or decades from now.

The beauty of gems is enhanced by cutting, shaping and polishing. Generally, beauty is considered to be of first importance, followed by durability and rarity. A gem may be rare and durable, but without beauty it would not be as valued. The beauty of a diamond is its brilliance and the absence of color. In contrast the beauty of an emerald or a ruby is their intense color. Wearing gemstone jewelry has been associated with wealth and status. If the gemstone is available in large quantity it is not perceived to be as valuable as a rare gem.

The source of color of the gem is it's reflection of light. A fine colored gem should not be so dark that it appears black, and not pale. It should have a good depth of color. A gem's luster is the reflection of light from the gem's surface. It is improved when the gems are polished. Brilliance is the reflection of light from the inside of the faceted gems.

Gems are derived from gemstones which are the raw material. Gemstones are primarily minerals. Some are rocks and others are the organic products of formerly living plants or animals. For example, ruby is a mineral, jade is a rock, and amber and pearls are organic.

Minerals are often valued as gemstones because they can be made into sparkling gems due to their transparency. They can have interesting optical properties and they can form large gemstones that are transparent. Some are symmetrical with repeating faces, and other minerals have no symmetry at all. When minerals are magnified for identification it is the crystal symmetry which is most helpful. There are seven systems of crystal symmetry. For example diamonds and garnets both have cubic symmetry and tourmaline has trigonal symmetry. Well formed minerals which are perfectly symmetrical are rare. This is due to the fact that they need perfect growing conditions and sufficient space to grow without being constrained.

The durability of a gemstone is derived by its toughness, stability and hardness. A gem's toughness is it's resistance to chipping, cracking and breaking. Stability refers to a gem's resistance to structural or chemical changes as a result of exposure to external elements. For example, pearls can be damaged by perfume or alcohol. A pearl necklace should always be worn after applying hairspray or perfume. Some amethyst may fade when exposed to sunlight. However, most gems made into jewelry can withstand normal wear with minimal attention. Hardness is the gem's resistance to scratching. The Mohs scale of hardness ranks 10 minerals in order based upon their ability to scratch one another. The scale is from 1 to 10 with a 1 being the softest mineral. Diamonds are rated a 10 on the Mohs scale, topaz is rated 8 and talc is rated 1.

The standard unit of mass for gems is a carat. The abbreviation for carat is ct. One carat (1 ct.) equals 0.2 grams, 5 carats (5 cts.) equals 1.0 grams. Although they sound similar the words carat and karat have different meanings. Karats refers to the unit of measurement of gold purity. Understandably, many people often confuse the two terms.