Hexagonaria is a genus of colonial rugose coral. Fossils are found in rock formations dating to the Devonian period, about 350 million years ago. [Wiki]
 
The polished head shows the beautiful, inner structure of the coral colony. Fossils of this genus also form Petoskey stones, the State Stone of Michigan.
 
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* Photos show all sides.
* Sizes, shapes and colors my vary due to being a natural stone, being hand-carved and/or difference in computer monitors.