What is "phenacite"

Phenacite \Phen"a*cite\, n. [Gr. ?, ?, impostor, deceiver.] (Min.) A glassy colorless mineral occurring in rhombohedral crystals, sometimes used as a gem. It is a silicate of glucina, and receives its name from its deceptive similarity to quartz.

n. (context mineralogy English) A clear glassy mineral, a silicate of glucina, occurring in rhombohedral crystals and sometimes used as a gem.