Gem stones are aesthetically appealing minerals that are often cut and polished, then used to make jewelry. Some rocks are considered gem stones, even though they are not minerals, because they are used in the same way for jewelry. Many are very valuable, although value fluctuates and is based upon rarity and demand.

Types
There are two primary types of gem stones--precious and semi-precious. The definition of these types has changed over time. Traditionally, precious gem stones were classified as such because of their rarity, value or ceremonial significance. Today, only four stones are considered precious: diamonds, rubies, emeralds and sapphires. All others are considered semi-precious.

Classification
Gem stones are classified based on a variety of characteristics. First, their chemical composition is examined. This composition may be purely chemical, a simple chemical compound, or a mixture of compounds and elements. Next, the crystal structure is determined. This structure is based upon how the stone is formed, the symmetry and number of faces of the stone. Optical characteristics such as color are also examined, as well as refractive index, clarity and dispersion. All of these elements determine the classification of gem stones.

Grading
There is no grading system for colored gem stones in existence that is accepted universally. However, there are a number of laboratories that grade gem stones based upon their own methodology. Attempts have been made to standardize the grading of gem stones. Seven of the most respected gem stone grading labs have created the Laboratory Manual Harmonisation Committee (LMHC), which exists to standardize wording and analytical methods used to grade gem stones.

Cutting
The majority of gem stones are cut and polished before being used in jewelry. Some, such as quartz, may remain in their natural form. Gem stones may be cut as cabochons, which are smooth dome shapes, or with a faceting machine that forms facets at exact angles on the surface of the gem stone. Opaque stones such as turquoise and opal are usually cut as cabochons. Grinding wheels are used to grind and shape the stone while polishing agents are used to smooth. Transparent gems are typically faceted to emphasize their inner fire and sparkle. Facets come in many shapes and special machines are used to cut and polish the gem stones in this manner.

Treatments
In addition to cutting and polishing, gem stones are also commonly treated to enhance their clarity and color. Treatments can directly affect the value of the stone. Some are widely accepted while others are not. Heat is used to improve both clarity and color. Radiation is also used to change or darken color in some gem stones. Waxing or oiling treatments are applied to gem stones with large concentrations of surface fissures, such as emeralds or turquoise. This treatment fills in the fissures and improves color and clarity.