crystal (sense A. 2a); (also) a piece of this; †a container made of this (obsolete).

A hexagonal prismatic crystal (mentioned by Pliny Nat. Hist. xxxvii. ix. 52).

A piece of such a substance, typically considered as a precious stone or as having magical properties.

As a count noun: a piece or specimen of this.

Quartz in crystalline form, rock crystal; a piece or single crystal of this, (later esp.) one used in a clock or other device.