Gypsum casts are empty spaces (or secondarily-filled cavities) in sedimentary rocks that match the monoclinic form of gypsum crystals (CaSO4á2H2O). They demonstrate that the original sediments were deposited in a supratidal evaporitic setting. Seawater-soaked sediments quickly dried out after the tides receded. Dissolved minerals in the water precipitated to form crystals in the sediments.
In the case of the gypsum casts shown below, the original gypsum crystals dissolved away (gypsum is fairly soluble), leaving crystal-shaped gaps in the rock (originally a dolomitic mud).
Gypsum casts (field of view ~2.2 cm across) in weathered silicified metadolostone (Kona Dolomite, Paleoproterozoic, 2.20-2.29 billion years) from northern Michigan (Co. Rd. 480 roadcut, Ragged Hills, SW of Marquette, Upper Peninsula of Michigan, USA).