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Spar is an old mining or mineralogy term used to refer to crystals that have readily discernible faces. A spar will easily break or cleave into rhomboidal, cubical, or laminated fragments with smooth shiny surfaces.
The various spar minerals were a historical term among miners and alchemists for any nonmetallic mineral akin to gypsum, known in Old English as spærstān, spear stone, referring to its crystalline projections. Thus, the word spar in mineralogy has the same root as "spear,".
Amongst miners the term "spar" today is frequently used alone to express any bright crystalline substance. Most frequently, spar describes easily cleaved, lightly colored nonmetallic minerals such as feldspar, calcite or baryte. Baryte (BaSO4), the main source of barium, is also called "heavy spar" (Greek "barys" means "heavy"). Calcite often forms the dogtooth spar crystals found in vugs and caves.