A mineral first discovered in Russia in 1780, at the time thought to be a variety of emerald. However, it as Webster 1913 says, is actually part copper and is commonly found in the same areas where copper is mined (West Africa, Chile, the United States, particularly Arizona). It is only a 5.0 on the Mohs hardness scale, so it's not much used in jewelry even though the gemstone is beautiful.

Di*op"tase (?), n. [Gr. = through + to see: cf. F. dioptase.] Min.

A hydrous silicate of copper, occurring in emerald-green crystals.

© Webster 1913.