Origin: Aude (France)
Description: Concentric layers around Brotia scheri. Oncoliths are microbialites with spherical or ovoid sedimentary structures of organic origin, formed by concentric layers of calcium carbonate. The formation of oncoliths is very similar to that of stromatolites, but developed on a base not fixed to the substrate. The structure is often formed from a nucleus, which can be a sedimentary particle or a shell fragment, on which establishes a microbial mat of cyanobacteria that induce precipitation and fixation of carbonate. 100% natural piece, cut and polished.
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