Many of the optical properties of dolomite are similar to those of calcite, including the high birefringence that produces distinctive changes in relief with rotation of the stage (note the differences between A and B, rotated 90 degrees with respect to one another).  The creamy high order white interference colours (C and D) are also identical to those of calcite.  Dolomite occurs  in marbles and carbonatites, but its most common occurrence is in sedimentary carbonate rocks, in which it is of secondary (diagenetic) origin, replacing calcite.  In such situations, dolomite is commonly euhedral to subhedral, occurring as masses of rhombs (A-D) or euhedral crystals projecting into pore spaces.  These textures are not commonly exhibited by calcite, which is typically anhedral.  Samples of dolostones are from the Lockport Formation, Beamsville, Ontario (A, B and C) and the Mid-Ordovician “Basal Beds”, Manitoulin Island, Ontario.  All views are 2.2 mm across.  A, B ppl, C, D x-nicols.