Rock Identifier
Fossil Coral (Petoskey Stone / Favosites / Charlevoix Stone) (Hexagonaria percarinata / Favosites (Cnidaria)) — fossil
fossil

Fossil Coral (Petoskey Stone / Favosites / Charlevoix Stone)

Hexagonaria percarinata / Favosites (Cnidaria)

Hardness: 3-4 on Mohs scale (composed mainly of calcite). Color: Gray, tan, and light brown with distinct, repeated cell structures or parallel line patterns. Luster: Dull to earthy in raw form, waxy to vitreous when polished. Structure: Colonial coral skeleton showing corallite structures.

Hardness
3-4 on Mohs scale (composed mainly of calcite)
Luster
Dull to earthy in raw form, waxy to vitreous when polished
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Physical properties

Hardness: 3-4 on Mohs scale (composed mainly of calcite). Color: Gray, tan, and light brown with distinct, repeated cell structures or parallel line patterns. Luster: Dull to earthy in raw form, waxy to vitreous when polished. Structure: Colonial coral skeleton showing corallite structures.

Formation & geological history

Formed during the Middle Devonian epoch (approx. 350-400 million years ago). Colonial coral organisms grew in warm, shallow prehistoric seas and were gradually replaced by calcium carbonate (calcite) and silica during fossilization.

Uses & applications

Used widely for lapidary work, jewelry (beads, pendants), decorative carvings, paperweights, and as a popular rockhound/fossil collection specimen.

Geological facts

When dry, the fossil patterns can appear faded, but wetting or polishing the surface brings out the intricate honeycomb or repeating corallite structure in striking detail.

Field identification & locations

Look for distinctive repeating geometric, lined, or honeycomb-like patterns embedded in a fine-grained, smooth pebble, common along Great Lakes beaches (especially Michigan) and gravel beds.

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