Polished specimen of fossil coral known as 'Petoskey stone'. Petoskey stone is a fossilized, prehistoric coral (Hexagonaria percarinata) that formed around 350-400 million years ago. They are known for their unique six-sided, honeycomb-like pattern and are the official state stone of Michigan. This is a good sized example that is polished on top and natural on the bottom. Ex. Mike Keim.