The U.S. Geological Survey is grateful to the following individuals for their assistance:
Harvey E. Belkin, Geologist, Geological Survey, U.S. Department of the Interior, Reston, VA.
Gordon T. Austin, "Gem Stones," _Mineral Commodity Summaries 1991_, Bureau of Mines, U.S. Department of the Interior, Washington, DC.
Robert E. Thaden, "Gem Stones," in _United States Mineral Resources_, Geological Survey Professional Paper 820, U.S. Government Printing Office, Washington, 1973, p. 247-250.
Jane Jenness, Minerals Information Office, U.S. Geological Survey, Washington, DC.
All photographs are courtesy of The Smithsonian Institution.
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End of the Project Gutenberg EBook of Natural Gemstones, by Various