Precious stones, also known as gemstones, are minerals or rocks that that become collectible when they are cut or faceted and polished. Many are commonly used in jewelry.
These stones can occur naturally, or they can be laboratory simulated. Natural stones come directly out of the ground or mine and are cut or faceted to the desired shape and size. They are usually the most popular among collectors and consumers, as well as the most expensive. Laboratory simulated versions are created in a lab. These manmade stones can still be considered precious stones, but are easier to come by and usually less expensive than their natural counterparts.
There are also organic gemstones, including amber, which is made from fossilized tree resin; and jet, which is a form of coal. There are some precious stones that are too soft or fragile to be used in jewelry, such as single-crystal rhodochrosite. Although they cannot be used for jewelry, they are exhibited in museums and sought by collectors because of their beauty.
Gemologists use technical specifications to differentiate between stones. The first consideration when identifying a gem is the chemical composition. Diamonds, for example, are made of carbon, and rubies are made of aluminum oxide. Most precious stones are actually crystals, which are classified by a crystal system as cubic, trigonal, or monoclinic.
These stones are also classified into different groups, species, and varieties. The ruby, for example, is the red variety of the species corundum, which belongs to the spinel or hematite group. The emerald, aquamarine, bixbite, goshenite, heliodor, and morganite, on the other hand, are all varieties of the mineral species beryl.
Separating precious stones into these types of groups helps to identify their composition, their color, and their origin. They also have a refractive index, dispersion, specific gravity, hardness, cleavage, fracture, and luster. Each of these features is important when assessing the value of the stone.