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,".
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Satin spar, less commonly atlas spar, is a trivial, partly obsolete name for fibrous varieties of the minerals gypsum, calcite, and aragonite. Satin spars have a characteristic satin (silky) luster. Despite their very low hardness (2 or 3 on the Mohs scale), satin spars are widely used as ornamental stones.
Spar is a general term for glassy, transparent to translucent minerals that cleave easily. Satin spars are not related to other minerals containing spar in their names, such as feldspar and fluorspar.
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