fossil

Fossiliferous Limestone / Petoskey Stone (Coral Fossil)

Hexagonaria percarinata / Calcite (CaCO3)

AI-generated identification · may be incorrect

Hardness: 3 on Mohs scale; Color: Grey, beige, light brown; Luster: Dull to earthy (pebble stage), vitreous when polished; Crystal Structure: Microcrystalline calcite (trigonal); Cleavage: Perfect rhombohedral in individual calcite grains; Specific Gravity: 2.71

Hardness: 3 on Mohs scale
Color: Grey, beige, light brown
Luster: Dull to earthy (pebble stage), vitreous when polished

Identified July 22, 2026More fossil →

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Physical properties

Formation & geological history

Formed during the Middle Devonian period (around 350 million years ago) from ancient rugose coral colonies living in warm, shallow tropical seas. Over time, silts and calcium carbonate buried the coral, replacing the organic matter with calcite limestone.

Uses & applications

Used for lapidary arts, jewelry, decorative polished stones, local souvenir crafts, and scientific study. High-grade unpolished or polished specimens are prized by fossil collectors.

Geological facts

The Petoskey stone is the official state stone of Michigan, USA. Its characteristic 'sunburst' or honeycomb pattern becomes dramatically more visible when wet or polished, revealing individual corallites.

Field identification & locations

Identified by the tiny pores and hexagonal corallite pattern visible on weathered surfaces. Commonly found along the beaches of Lake Michigan and Lake Huron, as well as gravel pits in northern Michigan. Wetted surfaces help bring out the fossil structure for easy field identification.