Coral (fossil)

Fossil coral, as used for beads and cabochons, is ancient coral whose original calcium carbonate skeleton has been replaced or filled by silica (chalcedony, opal and quartz) over millions of years. It is therefore a natural fossil made of silica minerals, not living-coral organic material. Geologists call it silicified coral when the coral's internal structure is preserved, and agatized coral when banded chalcedony (agate) fills the space the coral left.