fossil
Petoskey Stone (Fossilized Rugose Coral)
Hexagonaria percarinata
Hardness: 3-3.5 on Mohs scale; Color: Gray, tan, or brown; Luster: Dull (natural) to Vitreous (polished); Structure: Hexagonal colonial coral cells with radiating lines (septa); Composition: Calcite
- Hardness
- 3-3
- Color
- Gray, tan, or brown
- Luster
- Dull (natural) to Vitreous (polished)
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Physical properties
Hardness: 3-3.5 on Mohs scale; Color: Gray, tan, or brown; Luster: Dull (natural) to Vitreous (polished); Structure: Hexagonal colonial coral cells with radiating lines (septa); Composition: Calcite
Formation & geological history
Formed during the Devonian Period about 350 million years ago. These are fossilized colonial corals that lived in warm, shallow seas covering what is now the Michigan Basin. Glacial action eventually moved and distributed the fossils.
Uses & applications
Highly prized as a state stone for Michigan, used in lapidary work for jewelry, pendants, paperweights, and as collector specimens.
Geological facts
The name comes from Ottawa Chief Pet-O-Sega. While they appear as ordinary gray stones when dry, their unique hexagonal pattern 'blooms' and becomes strikingly visible when wet or polished.
Field identification & locations
Can be found along the shores of Lake Michigan and Lake Huron. Identify by the hexagonal honeycomb pattern. It is the official state stone of Michigan. Collectors often look for them after a storm when waves have turned up the shoreline.
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