sedimentary

Petoskey Stone (Hexagonaria coral fossil)

Hexagonaria percarinata

Hardness: 3-3.5 on Mohs scale. Color: Gray, tan, or brown, often showing reddish or pinkish hues due to iron staining. Luster: Dull to vitreous when polished. Crystal structure: Hexagonal/porous coral cell structure. Cleavage: None. Specific gravity: 2.7.

Hardness: 3-3
Luster: Dull to vitreous when polished

Identified June 19, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed during the Devonian period (approximately 350 million years ago) as a species of colonial rugose coral. These corals lived in warm, shallow seas that covered what is now the Great Lakes region. Over time, they were fossilized and later moved by glaciers during the Pleistocene epoch.

Uses & applications

Commonly used in jewelry, decorative items, and as a popular collector's item. It is the official state stone of Michigan.

Geological facts

The name 'Petoskey' comes from the Ottawa Indian Chief Pet-O-Sega. While they appear as ordinary gray stones when dry, their intricate hexagonal patterns become vividly visible when wet or polished.

Field identification & locations

Identified by the distinct 'sunburst' or hexagonal pattern of the coral cells. Mostly found along the shores of Lake Michigan and Lake Huron, particularly in the northern Lower Peninsula of Michigan. Tip: Look for them on beaches after a storm or when the stones are wet.

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