sedimentary
Petoskey Stone
Hexagonaria percarinata (Fossilized Rugose Coral)
Hardness: 3-4 (Mohs scale), Color: Gray/brown when dry, displaying a distinct hexagonal pattern when wet or polished; Luster: Dull to vitreous; Structure: Colonial coral skeletons consisting of hexagon-shaped cells (corallites).
- Luster
- Dull to vitreous
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Physical properties
Hardness: 3-4 (Mohs scale), Color: Gray/brown when dry, displaying a distinct hexagonal pattern when wet or polished; Luster: Dull to vitreous; Structure: Colonial coral skeletons consisting of hexagon-shaped cells (corallites).
Formation & geological history
Formed during the Devonian Period (approx. 350 million years ago) from colonial rugose corals. As the corals died, they were buried in sediment which eventually turned into limestone, petrifying the remains with silica and calcite.
Uses & applications
Used primarily in jewelry (beads, cabochons), home decor, and as popular collectibles. It is the official state stone of Michigan.
Geological facts
A Petoskey stone is actually a fossilized colony of ancient coral. The pattern is usually invisible until the stone is wet or polished, which brings out the intricate internal 'sunburst' or hexagonal structures of the individual coral polyps.
Field identification & locations
Identify by wetting the stone to reveal the hexagonal pattern. Mostly found on beaches and in glacial deposits in the northern Lower Peninsula of Michigan, specifically around Lake Michigan and Lake Huron.
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