ooid

thumb|Modern ooids from a beach on Joulter Cays, The Bahamas. thumb|Ooids on the surface of limestone; Carmel Formation (Middle [[Jurassic) of southern Utah, USA.]] thumb|A thin slice of calcitic ooids from the Carmel Formation, Middle Jurassic, of southern Utah, USA.

Ooids (, ) are small (commonly ≤2 mm in diameter), spheroidal, "coated" (layered) sedimentary grains, usually composed of calcium carbonate, but sometimes made up of iron- or phosphate-based minerals. Ooids usually form on the sea floor, most commonly in shallow tropical seas (around the Bahamas, for example, 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. Oolites usually consist of calcium carbonate; these belong to the limestone rock family. Pisoids are similar to ooids, but are larger than 2 mm in diameter, often considerably larger, as with the pisoids in the hot springs at Carlsbad (Karlovy Vary) in the Czech Republic. Ooids have been the subject of scientific research for centuries.