Gypsum is common in sedimentary rocks, especially in marine evaporite deposits.  It is colourless (A) and has low negative relief relative to epoxy and moderate negative relief relative to anhydrite, with which it commonly occurs.  It is typically anhedral, but tabular euhedral grains are elongate parallel to the c axis.  Gypsum has perfect {010} cleavage and good {100} cleavage (A), and has maximum interference colours of first order white to straw yellow, similar to quartz.  It is monoclinic and has inclined extinction.  Sample is from an unknown location.  Both views are 2.2 mm across.  A ppl, B x-nicols.