The Collaborative International Dictionary
Dioptase \Di*op"tase\, n. [Gr. ? = dia` through + ? to see: cf. F. dioptase.] (Min.) A hydrous silicate of copper, occurring in emerald-green crystals.
Wiktionary
n. An intense emerald-green to bluish-green copper cyclosilicate mineral.
Wikipedia
Dioptase is an intense emerald-green to bluish-green copper cyclosilicate mineral. It is transparent to translucent. Its luster is vitreous to sub- adamantine. Its formula is CuSiO·HO (also reported as CuSiO(OH)). It has a hardness of 5, the same as tooth enamel. Its specific gravity is 3.28–3.35, and it has two perfect and one very good cleavage directions. Additionally, dioptase is very fragile and specimens must be handled with great care. It is a trigonal mineral, forming 6-sided crystals that are terminated by rhombohedra.
Usage examples of "dioptase".
Thus green is the colour for troubles of the heart, with Monday the best day and dioptase or malachite the best stones.