Rock gypsum (a.k.a. gyprock) is a chemical sedimentary rock. It is an example of an evaporite - it forms by the evaporation of water (usually seawater) and the precipitation of dissolved minerals. Rock salt & rock gypsum often occur together in evaporitic successions. Rock gypsum is composed of the mineral gypsum (CaSO4·2H2O - hydrous calcium sulfate). Heating of gypsum or rock gypsum drives off the water, leaving only calcium sulfate behind (the mineral anhydrite). Adding water to anhydrite results in the formation of gypsum again.
Rock gypsum, unlike rock salt, does not have a salty taste, and is softer (H = 2) - it can be scratched with a fingernail. Rock gypsum’s color is often a mottled whitish-light grayish-light brownish. It is usually microcrystalline and powdery looking (it's much finer-grained than typical rock salt deposits). Rock gypsum superficially resembles chalk. Chalk is calcitic, and so will bubble in acid - rock gypsum does not bubble in acid.
Rock gypsum (Gyprock)