sedimentary
Fossilized Coral (Petoskey Stone type)
Hexagonaria percarinata (Order: Rugosa)
Hardness: 3-4 (Calcite-based) or 6.5-7 (if silicified); Color: Tan, beige, or grey; Luster: Dull to waxy; Texture: Hexagonal fossil pattern; Specific Gravity: 2.6-2.7
- Hardness
- 3-4 (Calcite-based) or 6
- Color
- Tan, beige, or grey
- Luster
- Dull to waxy
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Physical properties
Hardness: 3-4 (Calcite-based) or 6.5-7 (if silicified); Color: Tan, beige, or grey; Luster: Dull to waxy; Texture: Hexagonal fossil pattern; Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the fossilization of ancient rugose coral colonies during the Devonian period, roughly 350-400 million years ago, in warm shallow seas.
Uses & applications
Used for jewelry (cabochons), lapidary art, paperweights, and as a popular collector's item and state stone of Michigan.
Geological facts
This specimen shows the distinctive 'star' or hexagonal pattern characteristic of colonial rugose corals. While living, each hexagon was a polyp.
Field identification & locations
Identify by wetting the surface to see the hexagonal 'cells' more clearly. Commonly found on beaches of the Great Lakes, especially Lake Michigan.
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