Rock Identifier
Petoskey Stone (Hexagonaria percarinata (Fossilized Coral)) — sedimentary
sedimentary

Petoskey Stone

Hexagonaria percarinata (Fossilized Coral)

Hardness: 3-3.5 (Mohs scale); Color: Typically light grey to tan when dry, reveals dark grey/brown hexagonal patterns when wet or polished; Luster: Dull to waxy; Structure: Colonial coral fossils with individual corallites showing radiating lines; Cleavage: None (fractures irregularly); Specific gravity: ~2.7

Hardness
3-3
Luster
Dull to waxy
Identified More sedimentary →
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Physical properties

Hardness: 3-3.5 (Mohs scale); Color: Typically light grey to tan when dry, reveals dark grey/brown hexagonal patterns when wet or polished; Luster: Dull to waxy; Structure: Colonial coral fossils with individual corallites showing radiating lines; Cleavage: None (fractures irregularly); Specific gravity: ~2.7

Formation & geological history

Formed during the Devonian Period (approximately 350 million years ago) in warm, shallow saltwater seas. The prehistoric coral colonies were later covered by sediment and fossilized as calcite replaced the organic structures. Glacial action during the Pleistocene epoch broke these formations into the rounded stones found today.

Uses & applications

Primarily used for jewelry, lapidary arts, and as a popular souvenir/collector's item. In Michigan, it is the official state stone and is highly valued by rockhounds.

Geological facts

The 'Petoskey' name comes from an Ottawa Chief, Ignatius Petosega. While it looks like an ordinary grey rock when dry, the unique 'six-sided sun' pattern emerges instantly when the stone is moistened. It is actually a fossilized colonial rugose coral.

Field identification & locations

Identify in the field by looking for rounded, grey stones along shorelines; wet the stone to reveal the characteristic hexagonal honeycomb pattern. Most commonly found on the shores of Lake Michigan and Lake Huron in the northern Lower Peninsula of Michigan.

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