fossil

Petoskey Stone

Hexagonaria percarinata (Fossilized Rugose Coral)

AI-generated identification · may be incorrect

Hardness: 3.0-3.5 (Calcite) to 7.0 (if silicified); Color: Tan, gray, or brown when dry, distinct six-sided star patterns appear when wet; Luster: Dull to vitreous when polished; Cleavage: Rhombohedral (in calcite parts).

Hardness: 3
Luster: Dull to vitreous when polished

Identified July 2, 2026More fossil →

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Physical properties

Formation & geological history

Formed from ancient rugose corals that lived in warm, shallow Devonian seas roughly 350 million years ago. During the Pleistocene ice age, glaciers plucked these stones from the bedrock and deposited them as glacial till across the Great Lakes region.

Uses & applications

Primarily used as a state stone for Michigan, in jewelry (necklaces, rings), as decorative paperweightsRoot, and as a popular collector's item for beachcombers.

Geological facts

The Petoskey stone is the official state stone of Michigan. The name comes from an Ottawa Chief, Pet-O-Sega, meaning 'rising sun' or 'rays of hope', referring to the sun-like pattern of the coral.

Field identification & locations

Identify by looking for the hexagonal 'honeycomb' pattern. They are most easily found along the shores of Lake Michigan and Lake Huron, especially after a storm or in the spring when the ice thaws. Wetting the stone makes the pattern visible.