Petoskey Stone (Coral Fossil)

Physical Properties

Hardness: 3-4 (Mohs scale), Color: Brown to gray, Luster: Dull to waxy when unpolished, vitreous when polished, Structure: Distinctive hexagonal/circular pattern of individual coral polyps, Cleavage: None (conchoidal fracture).

Formation & Geological History

Formed during the Devonian Period (approximately 350 million years ago) from ancient coral reefs in shallow tropical seas that once covered what is now the midwestern United States.

Uses & Applications

Primarily used as jewelry, decorative lapidary items, and as a popular collector's item for hobbyists.

Geological Facts

It is the official state stone of Michigan. The name comes from an Ottawa Chief named Pet-O-Sega. The patterns visible on the surface are the fossilized skeletons of individual coral polyps.

Field Identification & Locations

Identify in the field by looking for the 'six-sided' or 'honeycomb' pattern that becomes much more visible when the stone is wet. Found mostly on beaches in Northern Michigan, especially near Petoskey and Traverse City.

Identified on: 4/26/2026

Mode: Standard