The proper name for fossil coral is "agatized coral" or "agatized fossil coral." Agate, with its silicon dioxide composition (SiO₂), is a variety of naturally occurring chalcedony--or microcrystalline quartz--rather than a type of coral. They are very grounding and improve blood and air circulation. Fossil coral is used for healing eye, skin and stomach illnesses. It is even believed to enhance longevity.