sedimentary

Petoskey Stone

Hexagonaria percarinata (Fossilized Rugose Coral)

AI-generated identification · may be incorrect

Hardness: 3-3.5 on Mohs scale. Color: Mottled gray, tan, or white with distinct hexagonal patterns. Luster: Dull to vitreous when polished. Crystal structure: Trigonal (calcite replacement). Cleavage: Perfect rhombohedral in calcite matrix.

Hardness: 3-3
Color: Mottled gray, tan, or white with distinct hexagonal patterns
Luster: Dull to vitreous when polished

Identified June 24, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed during the Devonian Period (approx. 350 million years ago) from the fossilized remains of ancient rugose corals. The corals lived in warm, shallow seas that covered present-day Michigan. Glacial movement during the last Ice Age plucked these fossils from the bedrock and deposited them as rounded pebbles.

Uses & applications

Primarily used for decorative purposes, lapidary arts, jewelry (cabochons), paperweights, and as a popular souvenir/collector's item.

Geological facts

It is the official state stone of Michigan. The unique 'eye' pattern is the fossilized skeleton of individual coral polyps. When dry, it looks like an ordinary gray rock, but when wet or polished, the pattern becomes vivid.

Field identification & locations

Identify in the field by looking for 'honeycomb' or 'star-burst' hexagonal patterns on rounded beach stones, especially along the shores of Lake Michigan and Lake Huron. It will react and fizz if a drop of vinegar (weak acid) is applied due to the calcite content.