fossil

Petoskey Stone (Fossilized Rugose Coral)

Hexagonaria percarinata

Hardness: 3 to 4 on the Mohs scale. Color: Light grey, tan, to brownish white with characteristic hexagonal dark-and-light eye patterns. Luster: Dull to earthy when unpolished, vitreous to silky when polished. Composition: Calcite (CaCO3).

Hardness: 3 to 4 on the Mohs scale

Identified July 22, 2026More fossil →

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Physical properties

Formation & geological history

Formed during the Devonian Period, approximately 350 million years ago, when tropical seas covered present-day Michigan. Extinct rugose coral colonies were buried under silt and mud, and replaced over geological time by calcite.

Uses & applications

State stone of Michigan, popular for lapidary work, jewelry (cabochons, pendants), decorative paperweights, carvings, and educational fossil collections.

Geological facts

The distinct pattern consists of individual coral polyps (corallites) packed tightly together in a hexagonal grid. In dry, unpolished condition, the pattern can be faint, but wetting it or polishing it brings out the intricate hexagonal honeycomb structure clearly.

Field identification & locations

Can be identified in the field by looking for a pebble or cobble displaying a repeating honeycomb or star-like hexagonal pattern, particularly around beach shores of Lake Michigan and Huron.

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