sedimentary

Petoskey Stone

Hexagonaria percarinata (Fossilized Rugose Coral)

Hardness: 3 to 3.5 (Mohs scale); Color: Gray to brownish-gray; Luster: Dull to vitreous when polished; Crystal Structure: Hexagonal patterns (corallites); Cleavage: None; Specific Gravity: Approx 2.7.

Hardness: 3 to 3
Color: Gray to brownish-gray
Luster: Dull to vitreous when polished

Identified June 30, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed during the Devonian Period (approx. 350 million years ago) as coral reefs in warm, shallow seas. As the coral died, it was buried under sediment and eventually fossilized into limestone, then weathered and smoothed by Pleistocene glacial activity.

Uses & applications

Predominantly used for jewelry, decorative carvings, and as a popular collector's item. It is the official state stone of Michigan.

Geological facts

The name 'Petoskey' comes from an Ottawa Indian Chief, Pet-O-Sega. The stone's distinctive 'six-sided star' or honeycomb pattern is only visible when the stone is wet or polished.

Field identification & locations

Commonly found on the shores of Lake Michigan and Lake Huron. Identify in the field by looking for a gray, smooth pebble that shows a faint honeycomb pattern when dipped in water.

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