Gypsum is a soft, white or gray mineral that forms in ancient seabeds when water evaporates and leaves behind calcium sulfate crystals. You've almost certainly encountered gypsum without knowing it: the white powder inside drywall (also called sheetrock or plasterboard) that makes up most interior walls in homes and schools is made from gypsum. When you see someone patching a hole in a wall with joint compound or spackle, that's gypsum too.

Gypsum is so soft you can scratch it with your fingernail, which makes it easy to grind into powder and mix with water. Ancient Egyptians discovered that when you heat gypsum, grind it up, and add water, it forms a paste that hardens as it dries. They used this material, called plaster, to create smooth walls inside pyramids. Modern builders still use the same basic process, though now they sandwich the gypsum between sheets of heavy paper to make drywall panels.

Gypsum also enriches soil in farming and appears in blackboard chalk, cement, and even some foods as a harmless additive. The largest gypsum crystal ever found, discovered in a Mexican cave, was longer than a school bus and clear as glass.