fossil

Fossilized Coral (Petoskey Stone type)

Hexagonaria percarinata (Fossilized Rugose Coral)

Hardness: 3-4 (Calcite-based), Color: Tan, gray, or brown with distinctive honeycomb/hexagonal patterns, Luster: Dull to waxy, Crystal Structure: Trigonal (as calcite replaced the organism), Cleavage: Perfect rhombohedral in calcite matrix.

Identified July 4, 2026More fossil →

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Physical properties

Formation & geological history

Forming during the Devonian Period (approx. 350-400 million years ago), these were living coral reefs in warm, shallow seas that covered parts of North America. They were later fossilized and shaped by glacial movement.

Uses & applications

Commonly used in jewelry, as decorative pocket stones, for educational displays, and as a popular state stone for collectors.

Geological facts

The pattern visible is the fossilized skeleton of colonial coral. When dry, it often looks like an ordinary limestone pebble, but when wet or polished, the intricate six-sided polyps become clearly visible.

Field identification & locations

Can be identified by the 'honeycomb' or star-like pattern on the surface. They are most famously found along the shores of Lake Michigan and Lake Huron, particularly after storms stir up the shoreline.

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