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
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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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