A type of sedimentary grain found in limestones. It has the appearance of small (generally 1-2mm) grains with concentric internal structure, formed by thin algal films formed as currents roll the grain back and forth.

Tiny round grains composed of concentric layers around a nucleus, usually calcium carbonate.

A small, rounded, light-colored concretion which forms as the mineral aragonite builds up successive layers around a nucleus such as a mineral grain or brine shrimp fecal pellet

A spherical grain of calcium carbonate, either aragonite or calcite. See the Ooid Formation tutorial for more details of their formation and origin.

Carbonate grains formed of thin, concentric rings of calcite precipitated around a quartz or shell fragment nucleus; diameter is less than 2 mm.

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.