5.1" Polished Oligocene Fossil Coral Sphere - Philippines

About Fossil Coral

Corals are marine invertebrates belonging to the phylum Cnidaria, most commonly found in warm, shallow oceans where they form extensive reef ecosystems. Individual coral animals, called polyps, secrete a hard external skeleton made primarily of calcium carbonate (CaCO₃) in the mineral form aragonite. Over time, successive generations of polyps build upon the skeletal remains of previous ones, producing massive reef structures that can persist for thousands to millions of years. Coral reefs play a critical role in marine biodiversity, coastal protection, and global carbon cycling.

Coral fossilization occurs through a series of geological and chemical processes after the coral organism dies. Initially, the soft tissues decay or are consumed, leaving behind the calcium carbonate skeleton. For fossilization to proceed, the skeleton must be rapidly buried by sediments such as sand, mud, or volcanic ash, which protect it from physical erosion and biological destruction. Over long timescales, mineral-rich groundwater percolates through the buried coral. During this process, original aragonite may be preserved or altered through diagenesis, commonly recrystallizing into the more stable mineral calcite. In some cases, the original skeletal material is partially or completely replaced by other minerals, such as silica, while retaining the coral’s original microstructure.

Fossilized corals provide valuable scientific information about Earth’s past environments. Because coral growth is sensitive to water temperature, chemistry, and sea level, coral fossils are widely used as indicators of ancient climates, ocean conditions, and plate tectonic movements. Their presence in sedimentary rocks far from modern oceans also offers evidence of historical changes in sea level and continental positioning over geological time.