sedimentary
Fossilized Coral (Petoskey Stone or Rugose Coral)
Antophyllum (Hexagonaria) or Scleractinia (CaCO3)
Hardness: 3-4 (Mohs scale), Color: Gray to tan, Luster: Dull to earthy, Structure: Visible calyx patterns (honeycomb or stars), Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
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Physical properties
Hardness: 3-4 (Mohs scale), Color: Gray to tan, Luster: Dull to earthy, Structure: Visible calyx patterns (honeycomb or stars), Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the replacement of organic coral structures with minerals (silica or calcite) in marine basin environments. Many date to the Devonian period (~350 million years ago).
Uses & applications
Used primarily for jewelry (cabochons), lapidary art, decorative paperweights, and as educational fossils for collectors.
Geological facts
Ancient coral reefs once covered much of the central United States; these fossils are remnants of warm, shallow tropical seas that preceded the formation of the Great Lakes.
Field identification & locations
Identify by honeycomb patterns or small circular pores on the surface. Commonly found along shorelines and in limestone quarries. Wetting the stone often reveals the hidden pattern.
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