Fossilized Coral | Petoskey Stone



close up image of the mineral fossilized coral



What is Fossilized Coral?

Fossilized coral is the result of fossilized coral colonies which date back approximately three hundred and fifty million years.  It can be found in a number of locations worldwide and some of the finest material comes from the shores of Lake Michigan in the USA and from Indonesia.  Part of an extinct species of coral that lived in the sea millions of years ago, hexagonaria as it's officially known is one of the most common varieties and the class known as hexagonaria percarinata is found close to Little Traverse Bay off Lake Michigan.

Widely used for lapidary purposes, the material from the USA is also known as Petoskey Stone after the city of Petoskey which lies on the south east shore of Little Traverse Bay.  The name came about because fossilized coral can be found there in great abundance and in 1965 Michigan adopted it as their state gemstone.



Formation of Fossilized Coral

Fossilized coral formed when sections of the ancient coral was gradually replaced by natural minerals including calcite and silica.  The corals had been growing in the oceans for almost five hundred million years and many thrived in warm shallow waters but over time they became buried under sediments as land surface moved and the oceans rose and fell.  Temperature and pressure from compaction resulted in deposits slowly turning into stone.  This process is known as petrifaction.



large oval shaped ladies pendant featuring a polished fossilized coral gemstone in a sterling silver setting



Three hundred and fifty million years ago the land that makes up Michigan was situated near the equator and at that point in the Devonian Period a large shallow sea covered a significant part of the United States. Much of Michigan was home to a large coral reef populated by various species of fish, trilobite and cephalopod. Although fossils of all of these creatures have been found, the most famous are the actual corals themselves.



close up photograph of fossilised coral



Fossilized coral was a mid-sized coral renowned for its honeycombed appearance.  Individual coral creatures known as polyps secreted a limey substance which created a corallite skeleton.  When the skeletons combined they formed large coral reefs that were composed of millions of individual creatures.  Over time parts of the reefs fossilized.



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Further Reading

Material from Indonesia
State Stone of Michigan