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A spherical stromatolite that usually forms around a fragment of detritus such as a piece of shell or a grain of sand.

[Greek onkos, mass; see oncology + -lite.]

on′co·lit′ic (-lĭt′ĭk) adj.
References in periodicals archive ?
Oncoid nuclei are usually one of the bioclasts mentioned above or intraclasts, and occasionally they show compound nuclei.
In the transition zone, 1 to 3 cm of oncoid packstone grade up into coarser microhermal packstone.
At the lamproite-oncolitic limestone contact, all the sparitic cement and the oncoid edges were re-emplaced by a new generation of carbonate crystals (Fig.
They exhibit stromatolitic, microphytic thrombolytic facies with sedimentary fills, oncoids, oncolites, pesoids, lithoclasts and intraclasts.
The one-meter tall stromatolites are marine in form, occur immediately adjacent to shore rather than in deeper water, are covered by a peat and soil with inter-head areas filled with a complex stratigraphy, are colonized by living freshwater mussels on their sides, appear to rim cenotes, and have oncoids in the sand flats surrounding the field.
The oncoids, as large as 2 cm, typically have a bryozoan, stromatoporoid or brachiopod shell fragment as nucleus, encrusted with Girvanella and Rothpletzella.
Also oncoids with tubular structure as fine micrite precipitated on filaments (calcified filaments) are seen (Fig.