Agatized fossil coral is a very ancient coral that has gradually turned into agate. This occurred when the silica in the salty ocean water hardened and replaced the limy coral. The coral skeletons appear as tiny flowers on the stone. their appearance may vary greatly in a single stone depending on weathering, oxidation and the original mineral content. The oldest fossil corals are from the Ordovican period, over 450 million years ago. Corals are marine animals with a sac like body, mouth tentacles and skeleton. It is the skeleton that is seen in the fossil. Some people theroize that agatized fossil coral was the inspiration for millefiori (thousand flowers) glasswork.
Fossil coral typically comes in gray, black, brown, yellow, white, and on rare occasions, red colors. the most detailed specimans of fossil coral come from Indonesia. Entire coral heads are often preserved and appear just as they did 20 million years ago. In 2008, Indonesian fossil coral, processed in China to a red color, began appearing in the market place. The process is similar to the one used to make agate a red carnelian color. Fossil coral can also be dyed red, but the color fades over time. Florida and Georgia, in the United States are also a good source of fossil coral. In fact, in 1979, Florida designated it as the state stone. Since it has become agatized, on the Moh scale, fossil coral has a hardness of 7 which makes it suitable for all kinds of jewelry.