ovoid or spherical crystalline deposit with a concentric or radial structure; most are composed of calcium carbonate, but some are composed of silica, siderite, calcium phosphate, iron silicate, or iron oxide. Oölite diameters range from 0.25 to 2 mm (0.01 to 0.08 inch), with most being in the 0.5- to 1-millimetre range; oölitic bodies with diameters greater than 2 mm are called pisolites (pisolite).

The term oölite has been applied both to the concretionary bodies and to the rock composed largely of such structures; to avoid ambiguity, these structures sometimes have been called oöids, oöliths, or ovulites and the term oölite reserved for the rock. The term also is used in an adjectival sense, such as oölitic limestone or oölitic chert. False oölites bear superficial resemblance to oölites but are devoid of a regular internal structure.

Calcareous oölites form where cold oceanic waters flow onto warm shallow banks, as in the Bahamas. The carbonate is precipitated on bits of shell, quartz grains, or other nuclei. They also are known to form in springs and caves, as cave pearls.