fossil

Petoskey Stone

Hexagonaria percarinata

Hardness of 3-4 on the Mohs scale, grayish-brown to dark gray color when dry (darker when wet), vitreous to dull luster, hexagonal corallite structures, specific gravity of approximately 2.6 to 2.7

Identified July 30, 2026More fossil →

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Physical properties

Hardness of 3-4 on the Mohs scale, grayish-brown to dark gray color when dry (darker when wet), vitreous to dull luster, hexagonal corallite structures, specific gravity of approximately 2.6 to 2.7

Formation & geological history

Formed during the Devonian period, approximately 350 million years ago, when Michigan was covered by a warm, shallow tropical sea. They are fossilized colonies of extinct rugose corals.

Uses & applications

Popular in lapidary arts, used to make polished decorative objects, cabochons, jewelry, and as a collector's specimen.

Geological facts

The Petoskey stone is the official state stone of Michigan. The unique pattern on the stone is composed of a fossilized coral with distinct hexagonal shapes that show star-like patterns when wet or polished.

Field identification & locations

Easily identified by its characteristic pattern of six-sided corallites exposed on smooth, rounded pebbles. Frequently found along the shores of Lake Michigan and Lake Huron, as well as in glacial drift deposits throughout the northern Lower Peninsula of Michigan.