Gypsum, Calcium sulphate dihydrate, CaSO4 . 2H2O As described in other gypsum references, gypsum is very variable in colour and appearance. These two specimens are from the same location, Whyalla, Eyre peninsula, in South Australia. One specimen is a dark brown compact mass with two smooth faces, and the other specimen is much lighter in colour, and an untidy mass of elongated prismatic crystals.