Calcite occurs in a great variety of rock types, including sedimentary limestones, metamorphic marbles and igneous carbonatites and kimberlites.  It has very high birefringence, which means that its relief (in plane light) changes significantly as the stage is rotated (compare view B, rotated 90 degrees from A).  It is colourless in plane light, with rhombic cleavage visible in many examples and has maximum interference colours of high order white (C).  Twinning, discussed below, is visible in the grain at extinction in D, in which all the other grains exhibit high-order white interference colours.  Calcite is readily confused with the other common carbonate mineral dolomite, and some features that can be used to distinguish the two are discussed under “Dolomite” and “Calcite and Dolomite Twinning”.  Sample shown in A, B and C is a sparry shell fragment in a micritic limestone from the Ordovician Verulam Formation, Lakefield, Ontario.  D is sparry calcite from Gull River Formation, Millhaven, Ontario.  Views are 5.5 mm across.  A and B ppl, C and D x-nicols.