CHARLES D. WINTERS / SCIENCE PHOTO LIBRARY CHARLES D. WINTERS / SCIENCE PHOTO LIBRARY
Petoskey stone. Polished Petoskey stone, the fossilised remains of the Hexagonaria percarinata coral. These corals lived during the Devonian period (about 350 million years ago). They existed as colonies in the warm shallow seas which covered what is now the Michigan peninsula, USA. These colonies were made up of several individual animals. The dark spots were the mouth opening of an individual member of the colony known as a polyp. Around the mouth were tentacles which supplied the polyp with food, mainly plankton. The Petoskey stone is often used in jewellery and in 1965 became the state stone of Michigan, USA.
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