ooid

An ooid is a small, spherical crystalline deposit, typically ranging from 0.25 to 2 mm in diameter. Most ooids are composed of calcium carbonate, but they can also be made of silica, siderite, calcium phosphate, iron silicate, or iron oxide. These structures have a concentric or radial internal pattern.

The term "ooid" can refer to these individual bodies or to the rock composed of them, which is known as oolite. Ooids form when minerals precipitate onto a nucleus, such as a shell fragment or quartz grain, in environments where cold oceanic waters meet warm shallow banks, like the Bahamas. They can also form in springs and caves, creating structures known as cave pearls. Oolitic limestone, a type of oolite, is often quarried for building purposes.