What is 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 called?

phenacite — 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. (Webster's 1913.)

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