An Oolite (or an oolith) is a sphere consisting of several concentric layers of calcite (a form of calcium carbonate) that was created by precipitation in the supersaturated warm waters of shallow tropical seas. The term may also be applied to an oolitic rock (a rock formed from many compressed oolites).

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 2mm in diameter (1/12 of an inch).

Oolitic limestone, a limestone consisting of many compressed oolites, was formed in the United Kingdom during the Jurassic period, and forms the Cotswold Hills.

The name derives from the Greek word oon meaning egg.

See also

• Geologic timescale
• Geology of the United Kingdom
• Pearls - also formed from concentric layers of calcium carbonate