Multiple Oxides

Multiple oxides are compounds of two metallic elements with oxygen, as in chromite, FeCr₂O₄. Some of the multiple oxides are soft, but many are hard and can be used as gemstones when transparent and flawless. Chesterman, p. 415

Sources

• Chesterman, Charles W. The Audubon Society Field Guide to North American Rocks and Minerals, Alfred A. Knopf: New York (1987)