English: Oolitic sand from the Holocene deposits of Pyramid Lake in Washoe County, Nevada, USA. (Public display, Geology Department, Wittenberg University, Springfield, Ohio, USA).

Oolites are sand-sized (1/16 mm to 2 mm in size), well rounded, concentrically-layered calcite or aragonite grains (they are also known as ooliths and ooids). Oolites form by rolling back and forth on a shallow seafloor, or sometimes on a shallow lake bed, by wave action. Oolites are forming today on the Bahamas Platform and in Great Salt Lake, Utah, USA. Ancient, solidified deposits of oolites are called oolitic limestones.

Uncertainty exists about the specifics of the origin of oolites. Some researchers conclude that oolites form by completely inorganic chemical precipitation of CaCO₃ from water around some nucleus (a tiny shell or skeletal fragment or sediment grain). Other researchers conclude that the presence of bacterial films on oolite grain surfaces play a significant role in the precipitation of CaCO₃ layers. However, the undoubted presence of bacteria does not necessarily indicate a biogenic origin for oolites - bacteria are everywhere.