Calcite or calcspar mineral crystal.
calcspar definition by Webster's New World
Origin: translated, translation of German kalkspar ; from kalk, lime plush spar, spar
calcspar definition by American Heritage Dictionary
Origin: Partial translation of Swedish kalkspat : kalk, lime (from Old Swedish kalker, from Middle Low German kalk, from Latin calx, calc-, lime; see calx) + spat, spar (mineral).