Oolites are formed when a nucleus, typically a sand particle or shell fragment, accumulates a layer of calcite around it as currents roll it around on the sea bed. From time-to-time the growing oolite becomes buried beneath the surface, allowing the accumulated calcite to consolidate, with a fresh layer accumulating each time it returns to the surface. Oolites may grow up to 2.0 mm in diameter (1/12 of an inch). Spherical structures larger than 2.0 mm in diameter are called pisolites.
Oolites also occur in Chert, Dolomite, and Hematite. The oolitic dolomite and chert may result from the replacement of the original texture in limestone. Oolitic hematite occurs at Red Mountain near Birmingham, Alabama along with oolitic limestone.
Oolitic limestone, a Limestone consisting of many compressed oolites, was formed in the United Kingdom during the Jurassic period, and forms the Cotswold Hills. A particular type, Bath Stone, gives the buildings of the World Heritage City of Bath, England its distinctive appearance.
The name derives from the Greek word oon meaning egg.