fossil

Fossil Coral (Petoskey Stone / Rugose Coral)

Hexagonaria percarinata (or related colonial Rugosa coral) in calcite/limestone matrix (CaCO3)

AI-generated identification · may be incorrect

Hardness: 3 to 3.5 on the Mohs scale; Color: Tan, beige, brown, and cream with distinctive radiating starburst-like corallite patterns; Luster: Vitreous to dull, glossy when polished; Crystal structure: Trigonal (calcite); Cleavage: Perfect rhombohedral in calcite portions; Specific Gravity: ~2.7

Hardness: 3 to 3
Luster: Vitreous to dull, glossy when polished

Identified October 3, 2026More fossil →

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Physical properties

Formation & geological history

Formed during the Devonian Period (approximately 350-400 million years ago) when warm, shallow saltwater seas covered the region. Colonial rugose corals were buried by fine sediment and gradually permineralized by calcite and silica-rich waters, turning into fossiliferous limestone.

Uses & applications

Prized as decorative ornamental objects, polished spheres and eggs, cabochons for jewelry, paperweights, and popular collector fossils.

Geological facts

A specific variety of this fossil coral found in Michigan is called the Petoskey Stone, which was designated the official state stone of Michigan in 1965. When dry and unpolished, the distinctive corallite pattern is often hard to see, but it becomes strikingly clear when wet or polished.

Field identification & locations

Identified by the interlocking hexagonal or circular corallites showing radiant septa radiating from central points. Highly abundant along the shores of Lake Michigan, Lake Huron, and similar fossil coral deposits worldwide (such as in Indonesia and Morocco).