Chalk is a soft, porous form of limestone, composed mainly of calcium carbonate (CaCO₃, see calcium) with varying amounts of silica, quartz, feldspar, or other impurities. Chalk is generally gray-white or yellow-white and is derived chiefly from the remains of small marine organisms. Embedded flint nodules are commonly found in chalk beds. Chalk is widely used in lime and cement manufacture and as a fertilizer. It is also used in cosmetics, crayons, and oil paints; school chalk is today usually made from chemically-produced calcium carbonate.