As fossils, corals are found worldwide in sedimentary rocks. Based on these fossils, we know that the corals began their long evolutionary history in the Middle Cambrian period, over 510 million years ago. In Kansas, they were deposited from about 315 to 250 million years ago. Nice coral fossils are found in Pennsylvanian and Permian rock formations. Although corals are the main reef builders in modern oceans, not all corals build reefs. In addition to the corals, which are called framework organisms, other organisms contribute to the formation of reefs. For example, modern reefs are inhabited by binding organisms (such as encrusting algae) and filler organisms (such as snails, bivalves, and sponges), whose skeletons fill in the spaces in the reef after death.
Two groups of corals were important inhabitants of the Pennsylvanian and Permian seas--tabulate and rugose corals. Tabulate corals were exclusively colonial and produced calcium carbonate skeletons in a variety of shapes: moundlike, sheetlike, chainlike, or branching. Tabulate corals get their name from horizontal internal partitions known as tabulae. Some tabulate corals were probably reef builders..
The coral can be cut and placed into very fine jewelry, such as this Black Coral ring. However, coral is best used in earrings and pendants, as its softness can allow it to be easily damaged in jewelry such as rings, unless properly protected.
And of course in todays market place we have all kinds of fakes and man made gemstones. Although natural, colored coral fossil stones are normally found to have been helped a bit with some colorations from man.