'''Multiple oxides''' are [[compounds]] of two [[metallic]] [[elements]] with [[oxygen]], as in [[chromite]], FeCr<sub>2</sub>O<sub>4</sub>. Some of the multiple oxides are soft, but many are hard and can be used as [[gemstones]] when [[transparent]] and [[flawless]]. Chesterman, p. 415
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=Sources=
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*Chesterman, Charles W. ''The Audubon Society Field Guide to North American Rocks and Minerals'', Alfred A. Knopf: New York (1987)