This sample of gypsum is on display at the Smithsonian Museum of Natural History.
Gypsum is a sulfate mineral, containing calcium and sulfur. It is widely used as a material for making drywall boards for house construction (sheetrock). It is also used for making plaster of Paris.
Gypsum is one of the softest minerals, measuring 2 on the Mohs hardness scale. It can be scratched with a fingernail.