Oncolite

[3] The oncoids often form around a central nucleus, such as a shell fragment,[4] and a calcium carbonate structure is deposited by encrusting microbes.

Oncolites are indicators of warm waters in the photic zone, but are also known in contemporary freshwater environments.

[6] The appearance of recent or near-recent freshwater oncoids has been documented in two rivers in Bavaria: the Alz, whose source is the Chiemsee, and the Moosach, near Freising.

[7][8][9] Modern oncoids also grow in some springs in Atacama Desert in South America.

In one particular case a system of oncoids have been observed interface between Salar de Antofalla and an adjacent wetland.