Fossil Coral is a natural gemstone that is created when prehistoric coral is gradually replaced with agate. Corals are marine animals and it is their skeletons that are fossilized and preserved. The entire process can take over 20 million years. They originated over 240 million years ago. Corals live in marine water, at most depths and latitudes. They have been found in water 6000 metres deep, but are most common at depths of less than 500 metres