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U.S. Department of the Interior U.S. Geological Survey
Natural Gemstones
Searching for gemstones in the United States is a popular recreational activity for collectors and hobbyists
Natural gemstones
Diamond, corundum , beryl , topaz, and opal are generally classed as precious stones. All other gemstones are usually classed as semiprecious.
Geologic environment
Gemstones are not plentiful. Gemstones do not form "ore" deposits in the normal sense.
Gems, when present at all, tend to be scattered sparsely throughout a large body of rock or to have crystallized as small aggregates or fill veins and small cavities.
Even stream gravel concentrations tend to be small--a few stones in each of several bedrock cracks, potholes, or gravel lenses in a stream bed.
The average grade of the richest diamond kimberlite pipes in Africa is about 1 part diamond in 40 million parts "ore." Kimberlite, a plutonic igneous rock, ascends from a depth of at least 100 kilometers to form a diatreme . Moreover, because much diamond is not of gem quality, the average stone in an engagement ring is the product of the removal and processing of 200 to 400 million times its volume of rock.
Gemstones occur in most major geologic environments.
Each environment tends to have a characteristic suite of gem materials, but many kinds of gems occur in more than one environment. Most gemstones are found in igneous rocks and alluvial gravels, but sedimentary and metamorphic rocks may also contain gem materials.
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