Petoskey Stone

Physical Properties

Hardness: 3.0-3.5 (Mohs scale); Color: Gray to tan/brown; Luster: Dull (natural) to Vitreous (polished); Crystal Structure: Hexagonal patterns of the individual coral polyps (corallites) with radiating lines (septa); Specific Gravity: approx 2.7

Formation & Geological History

Formed through the fossilization of ancient rugose coral. These corals lived in warm, shallow seas during the Devonian Period, approximately 350+ million years ago. Over time, the coral skeletons were buried and replaced by the mineral calcite (calcification).

Uses & Applications

Primarily used for jewelry (pendants, rings), decorative ornaments, lapidary art, and as a popular collector's specimen. It is also the state stone of Michigan.

Geological Facts

The 'sun' or 'eye' pattern visible on the stone is the skeletal structure of a single coral polyp. The name 'Petoskey' comes from an Ottawa Chief named Ignatius Petosegay. The stones are most easily found on beaches when wet, as the pattern 'disappears' when the stone is dry and unpolished.

Field Identification & Locations

Identify in the field by searching for gray stones with a distinct hexagonal 'honeycomb' pattern. They are most commonly found on the shores of Lake Michigan and Lake Huron in the Michigan Basin. Collectors should look for distinct, unbroken coral patterns.

Identified on: 4/18/2026

Mode: Standard