Fossil Coral

Fossil coral forms when a buried coral's calcium carbonate skeleton is slowly dissolved, for example by acidic groundwater, and replaced or filled by dissolved silica. In Miocene corals from the island of Sumatra, Indonesia, studied by Spanish researchers, the carbonate was replaced by opal, cavities were cemented with opal and quartz, and the silica came from interbedded volcanic ash and rocks. Florida's agatized coral comes mainly from Tampa Bay, the Econfina River and the Withlacoochee and Suwannee river beds; a Florida Museum specimen of Siderastrea coral lived about 34–28 million years ago.

In silicified coral, the silica copies the coral cell by cell, so the walls and radiating septa of the individual corallites show as star patterns. In agatized coral the internal structure dissolved first, and the mould is filled with colourful banded chalcedony. Because it is silica, fossil coral is a different material from the calcium carbonate of precious coral.

Agatized coral has interested mineral collectors since the 19th century. In 1979 the Florida Legislature designated agatized coral the official state stone of Florida.