In geology, ooids are small (< 2 mm) spheroidal "coated" (layered) grains, usually composed of calcium carbonate, but sometimes made up of iron or phosphate minerals. Ooids usually form on the sea floor, most commonly in shallow tropical seas, for example on the Bahama Platform, or in the Persian Gulf. After being buried under additional sediment, these ooid grains can be cemented together to form a sedimentary rock called an oolite.