Hexagonaria alpenensis from Michigan

Hexagonaria is a common colonial rugose coral in the middle Devonian. It's also the source of the "Petosky stones" that one sees for sale or can find on the beaches of northern Michigan. Here is a typical example of a "Petosky stone" in the unpolished state.

This stone was cut but not polished to show the coral pattern that is hidden inside.

When you wet it you can see the coral pattern more clearly. Often these stones are polished and used for decoration.

The piece above had been washed and naturally tumbled by the surf of Lake Huron. Here is very good example of what a small intact colony looks like without the tumbling.

The underside show the wrinkled epitheca surface where it contacted the muddy bottom nicely.

I'm not sure which formation any of the above specimens came from. The "Petosky stones" were beach float while the colony was unlabelled save for "Michigan, Devonian". They all most likely originate in the middle Devonian (Givetian stage).