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Molecular Structure
According to Dave Jessey and Don Tarman at the California State Polytechnic University at Pomona's Department of Geology, a mineral's molecular structure (the way its molecules are lined up) is crystalline. Crystalline means the molecules form a specific shape. For example, molecules may form in a perfect cube, with each corner of the cube containing a molecule. A nonmineral has no crystalline structure. Rather, the molecules are just randomly packed tightly together. Since all gems are minerals, the molecular structure of a gem is crystalline. The shape of the molecular structure varies with the type of mineral.
Opacity
Many minerals are opaque. This means you cannot see through them. The chief reason many minerals are not made into gems is that the opaqueness of the mineral is not suitable for gems. According to Web Minerals, graphite is an opaque mineral. Graphite is the chief component of the "lead" in a pencil. While graphite is a very useful industrial mineral, it is not considered a gem. Many other minerals also have industrial uses but are not considered gems because they are opaque. Talc, another mineral, is chalky white to translucent. It is used to make baby powder, but not gems.
Transparency and Faceting
Gems have different colors and tints. However, all share the common trait of being transparent to slightly translucent. Consumers prefer gems to be transparent. Transparency means you can see through it. Gem cutters take advantage of the transparency factor when cutting angled faces, called facets, on the gemstones. A rough gemstone is not very appealing; many times it looks like just a rock. Facets make the gem far more appealing. Faceting is not done randomly. Angles of the facets are carefully planned in order to bring out the stone's best qualities. The objective is to make the gem sparkle and reflect as much light as possible. According to Fred Van Sant of the United States Faceters Guild, gemcutters consider a variety of factors when determining the best angles to cut. These factors include the color and the type of stone. The correct angles are determined to maximize the reflectance factor of captured light.
Four C's
The Gemological Institute of America (GIA) compares the "four C's" when looking at gems, particularly diamonds. These are carat, color, clarity and cut. Carat refers to how big the gem is, or its weight. Color is what tint the gem has, such as a yellow or a blue tint. Clarity refers to how pure the gem is. All natural gemstones have slight deposits of other minerals in them. These deposits are called inclusions. For example, a diamond may have a speck or two of coal in it that never crystallized. Cut refers to what type of facets were cut on the gem. The GIA has grading scales for all four Cs. The GIA appraises gems based upon a combination of all four grades.
Supply and Demand Factors
The economic forces of supply and demand have a great deal to do with a mineral's final price. For example, talc is abundant, so its price is very low. It can be purchased by the ton out of quarries. Diamonds, on the other hand, are very rare. It takes a great deal of mining just to find one diamond. The demand for diamonds is related to the quality of the gems, based on the four C's. For example, many diamonds are too poor in quality to be used as gems, so they are used in tools, such as grinding wheels. Another factor in the price is the amount of work needed to turn a rough stone into a gem. The cost of labor must be factored in, since turning a rough stone into a gem requires a great deal of skill.