Many building materials - concrete, tiles, brick, glass, paint, plaster, and drywall - contain rocks or components derived from minerals. Quarries, or open-pit mines, produce crushed rocks, gravel, and sand of different grain sizes, known as aggregates. Coarsely crushed rocks and gravel are mixed with cement, a binding material that holds the aggregate in place to form concrete. The ancient Romans invented concrete, but after their empire fell, concrete technology was forgotten until the 18th century. Limestone, a sedimentary rock, and gypsum, a mineral in sedimentary deposits, are two key ingredients of cement. Sand and smaller particles of crushed rock go into making bricks. Finely ground gypsum is filler in paint, plaster, and drywall. While different types of glass used in buildings may have specialized ingredients and coatings, they are all mostly silica, or melted quartz sand.