Oncolites are sedimentary structures formed out of oncoids, which are layered spherical growth structures formed by cyanobacterial growth. Oncolites are very similar to stromatolites, but instead of forming columns they form approximately spherical structures. Often the oncoids form around a central nucleus, such as a shell particle, and the calcium carbonate

Calcium carbonate

Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

 structure is precipitated by encrusting microbes. Oncolites are indicators of warm waters in the photic zone

Photic zone

The photic zone or euphotic zone is the depth of the water in a lake or ocean that is exposed to sufficient sunlight for photosynthesis to occur...

, but are also known in contemporary freshwater environments. These structures rarely exceed 10 cm in diameter.