Jewelry is a form of personal adornment, manifesting itself as necklaces, rings, brooches, earrings and bracelets. Jewelry made from any material, usually gemstones, precious metals or shells. Factors affecting the choice of materials include cultural differences and the availability of the materials.

A gemstone is a piece of mineral, which, in cut and polished form, is to make jewelry or other adornments. However, certain rocks and organic materials are not minerals, but are for jewelry, and are therefore often gemstones as well. Most gemstones are hard, but some soft minerals are in jewelry because of their luster or other physical properties that have aesthetic value. Rarity is another characteristic that lends value to a gemstone. Apart from jewelry, from earliest antiquity until the 19th century engraved gems and hard stone carvings such as cups were major luxury art forms the carvings of Carl Faberge were the last significant works in this tradition. The traditional classification in the West begins with a distinction between precious and semi-precious stones; similar distinctions are in other cultures. In modern usage, the precious stones are diamond, ruby, sapphire and emerald, with all other gemstones being semi-precious. Other stones are by their color, translucency and hardness. The traditional distinction does not necessarily reflect modern values, for example, while garnets are relatively inexpensive, a green garnet called Tsavorite, can be far more valuable than a mid-quality emerald. Another term for semi-precious gemstones used in art history and archaeology is hard stone.

In modern times gemologists, who describe gems and their characteristics using technical terminology specific to the field of gemology, identify gemstones. The first characteristic a gemologist uses to identify a gemstone is its chemical composition. For example, diamonds are made of carbon and rubies of aluminum oxide. Next, many gems are crystals which are classified by their crystal system such as cubic or trigonal or monoclinic. For example diamonds, which have a cubic crystal system, are often as octahedrons.

Gemstones are into different groups, species, and varieties. Ruby is the red variety of the species corundum, while any other color of corundum is sapphire. Emerald (green), aquamarine (blue), red beryl (red), goshenite (colorless), heliodor (yellow), and morganite (pink) are all varieties of the mineral species beryl.

Gems are in terms of refractive index, dispersion, specific gravity, hardness, cleavage, fracture, and luster. They may exhibit pleochroism or double refraction. They may have luminescence and a distinctive absorption spectrum.

There is no universally accepted grading system for gemstones. Diamonds are using a system developed by the Gemological Institute of America (GIA) in the early 1950s. Historically, all gemstones were using the naked eye. The GIA system included a major innovation: the introduction of 10-x magnification as the standard for grading clarity. Other gemstones are still using the naked eye.

A mnemonic device, the color, cut, clarity and carat, has been to help the consumer understand the factors used to grade a diamond. With modification, these categories can be useful in understanding the grading of all gemstones. The four criteria carry different weight depending upon whether they are to colored gemstones or to colorless diamond. In diamonds, cut is the primary determinant of value, followed by clarity and color. Diamonds are to sparkle, to break down light into its constituent rainbow colors chop it up into bright little pieces and deliver it to the eye (brilliance). In its rough crystalline form, a diamond will do none of these things; it requires proper fashioning cut. In gemstones that have color, including colored diamonds, it is the purity and beauty of that color that is the primary determinant of quality.

Physical characteristics that make a colored stone valuable are color, clarity to a lesser extent cut, unusual optical phenomena within the stone such as color zoning, and Astoria (star effects). The Greeks, for example, greatly valued Astoria in gemstones, which were as a powerful love charm, and Helen of Troy was to have worn star-corundum.

Aside from the diamond, the ruby, sapphire, emerald, pearl and opal have also been precious. Up to the discoveries of bulk amethyst in Brazil in the 19th century, amethyst was a precious stone as well, going back to ancient Greece. Even in the last century, certain stones such as aquamarine, peridot and cat's eye have been popular as precious.

Many gemstones are in even the most expensive jewelry, depending on the brand name of the designer, fashion trends, market supply, treatments etc. Nevertheless, diamonds, rubies, sapphires and emeralds still have a reputation that exceeds those of other gemstones.

Rare or unusual gemstones, generally meant to include those gemstones, which occur so infrequently in gem quality that they are scarcely except to connoisseurs, include andalusite, axinite, cassiterite, clinohumite and red beryl.

Gem prices can fluctuate heavily. In general, per carat, prices of larger stones are higher than those of smaller stones, but popularity of certain sizes of stone can affect prices.