Gemstones are minerals valued for their beauty including color, transparency, and luster or brilliance. Color is sometimes dependent on crystal structure and bond type, but it most frequently is related to chemical composition. Color is often characterized by the presence of transition elements either directly related to the composition, such as the blue or green of copper-containing minerals or as impurities that impart color to colorless minerals such as quartz.  The basic structures of minerals are frequently silicon oxides, which have already been discussed, or a form of Al 2 O 3 , corundum. It is a hexagonal closest packed arrangement of O 2– ions with Al 3+ ions in two-thirds of the octahedral holes. The oxide ions are stacked in the A-B-A-B arrangement, and the aluminum ions fall into those holes that allow them to touch six oxide anions.

Field Trips

In areas where possible, a trip to a working mine would be valuable. Many cities have rock and mineral displays in museums.

Knowledgeable Individuals

The earth science or geology teacher or local college professor Rock hobbyists Members of the local geological society Lapidarist—a collector or dealer in gems

Other

1. A little item that can be purchased in a toy store and uses the crystallization concept is called the Magic Tree. It uses a cardboard three-dimensional tree and a powdery compound (like copper sulfate) to make crystals grow from an evaporating solution. It is manufactured by New Tomorrow, 7251ĘGarden Grove Blvd. #E, Garden Grove, CA 92641.
2. An article in ChemMatters (published by the Education Division of the American Chemical Society) entitled "Growing Diamonds" (April, 1990, pp. 10-13) describes the  crystalline nature of diamond as it relates to graphite and presents some interesting comparisons between the two. In this same issue, the feature "Back Burner" also uses diamonds as its theme.
3. An essay in Aldo Leopold’s Sand County Almanac (1966) New York: Oxford University Press is especially fascinating as it describes the way in which an atom, called "X," cycles through the biosphere as a part of a much larger mineral cycle. The essay "Odyssey" is found in Part 2 (pp. 104-108).
4. The October 5, 1990 issue of Science (pp. 25-26) has a interesting article about the production of isotopically pure diamond—the best room temperature heat conductor ever made. Controversy between an amateur scientist and General Electric about the origin of the idea is described.
5. Mica is a sheet silicate that can be cleaved to make large translucent sheets.   These sheets were used as window closures prior to the advent of plate glass.
6. The brilliance of gemstones depends largely on their high refractive index.   Because diamond’s refractive index is so large, light rays exhibit such small angles of refraction that the rays often fail to leave the material, but will be reflected back into it. This phenomenon is known as total reflection. It can be achieved in a diamond if the stone is properly cut. When a diamond exhibits total reflection, none of the light entering the face of the stone will be able to escape out the back, but will be totally reflected back to the face, imparting a brilliance and sparkle to diamond that has only been matched recently by synthetic gems of almost equally high refractive index.