With the exception of quartz, calcite is the most widely distributed of minerals, whilst in the beautiful development and extraordinary variety of form of its crystals it is surpassed by none. In the massive condition it occurs as large rock-masses (marble, limestone, chalk) which are often of organic origin, being formed of the remains of molluscs, corals, crinoids,& c., the hard parts of which consist largely of calcite. Calcite has long been known under the names calcareous spar and calc-spar, and the beautifully transparent variety called Iceland-spar had been much studied. The strong double refraction and perfect cleavages of Iceland-spar were described in detail by Erasmus Bartholinus; the study of the same mineral led Christiaan Huygens to discover in 1690 the laws of double refraction, and E.L. Malus in 1808 the polarization of light.