ONC111.
Fossil Oncolite polished slab.
From the Miocene "Trealla Limestone" north of Carnarvon, Western Australia .
Age is approx. 15 million years.
Dimensions at widest points approx. 140 mm x 100 mm x 6.5 mm thick.
Weighs 204 grams.
Oncolite is a limestone composed largely of oncoids—rounded, coated grains that grow as thin layers of lime mud accumulate around a tiny nucleus (such as a sand grain or shell fragment) under microbial influence. Formed in shallow, sunlit waters with gentle movement, oncolites often display attractive concentric or wavy banding and are classic indicators of ancient shoreline and lagoon environments.
