sedimentary
Petoskey Stone
Hexagonaria percarinata (Fossilized Rugose Coral)
Hardness: 3-4 (Mohs scale); Color: Gray to tan when dry, reveals hexagonal pattern when wet; Luster: Dull to vitreous when polished; Crystal Structure: Microcrystalline calcite replacing organic coral structure; Cleavage: None (fractures); Specific Gravity: 2.7
- Hardness
- 3-4 (Mohs scale)
- Color
- Gray to tan when dry, reveals hexagonal pattern when wet
- Luster
- Dull to vitreous when polished
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Physical properties
Hardness: 3-4 (Mohs scale); Color: Gray to tan when dry, reveals hexagonal pattern when wet; Luster: Dull to vitreous when polished; Crystal Structure: Microcrystalline calcite replacing organic coral structure; Cleavage: None (fractures); Specific Gravity: 2.7
Formation & geological history
Formed during the Devonian Period (approx. 350 million years ago) when Michigan was covered by a warm, shallow sea. These are fossilized rugose corals that were later moved and smoothed by glacial action during the Pleistocene ice ages.
Uses & applications
Primarily used for jewelry, lapidary art, decorative paperweights, and as a popular collector's item; it is the official state stone of Michigan.
Geological facts
The unique 'sunburst' or hexagonal pattern is actually the fossilized skeleton of individual coral polyps. The name comes from Ottawa Chief Pet-O-Sega, meaning 'Rising Sun' or 'Rays of Hope.'
Field identification & locations
Identifier: Look for the distinctive six-sided 'honeycomb' pattern that becomes much clearer when the stone is wet. Location: Found primarily on the shores of Lake Michigan and Lake Huron in the northern Lower Peninsula of Michigan.
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