Key difference: Gypsum is a very soft mineral composed of calcium sulfate dehydrate. Limestone is a type of sedimentary rock. Gypsum is a very soft mineral composed of calcium sulfate dehydrate. It has the chemical formula of CaSO 4 ·2H 2 O. It is mainly deposited from lake and sea water, as well as in hot springs, from volcanic vapors, and sulfate solutions in veins in sedimentary rocks.