sedimentary
Fossilized Coral (Petoskey Stone or similar)
Hexagonaria percarinata (Fossilized Rugose Coral)
Hardness: 3-4 (calcite-based); Color: Grey, tan, or brown with white cellular patterns; Luster: Dull to earthy when raw, vitreous when polished; Structure: Hexagonal or colonial coral septa patterns.
- Hardness
- 3-4 (calcite-based)
- Color
- Grey, tan, or brown with white cellular patterns
- Luster
- Dull to earthy when raw, vitreous when polished
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Physical properties
Hardness: 3-4 (calcite-based); Color: Grey, tan, or brown with white cellular patterns; Luster: Dull to earthy when raw, vitreous when polished; Structure: Hexagonal or colonial coral septa patterns.
Formation & geological history
Formed through the permineralization of ancient coral reefs, primarily during the Devonian period (approx. 350-400 million years ago) when warm shallow seas covered much of the northern hemisphere.
Uses & applications
Used primarily for fossil collecting, lapidary arts, jewelry (cabochons), and educational specimens.
Geological facts
While the note in the image says 'Gneiss', the visible 'honeycomb' or 'web-like' pattern is diagnostic of fossilized colonial coral. If found in Michigan, it is famously known as a Petoskey Stone.
Field identification & locations
Look for distinctive geometric patterns (corallites) that appear like lace or a honeycomb. They are often found in gravel pits, on beaches of the Great Lakes, or in limestone quarries.
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