sedimentary
Fossil Coral (Petoskey Stone or Favosites)
Favosites (specifically Hexagonaria percarinata if silica-replaced Michigan type)
Hardness: 3-4 (calcite) or 6.5-7 (if silicified/agatized); Color: Tan, brown, or grey; Luster: Dull to earthy; Structure: Colonial hexagonal chambers (corallites); Specific gravity: Approx 2.6-2.7.
- Hardness
- 3-4 (calcite) or 6
- Color
- Tan, brown, or grey
- Luster
- Dull to earthy
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Physical properties
Hardness: 3-4 (calcite) or 6.5-7 (if silicified/agatized); Color: Tan, brown, or grey; Luster: Dull to earthy; Structure: Colonial hexagonal chambers (corallites); Specific gravity: Approx 2.6-2.7.
Formation & geological history
Formed from the fossilized skeletons of extinct colonial rugose or tabulate corals in shallow, tropical seas during the Devonian period (approx. 350-400 million years ago). Through permineralization, minerals filled the porous structures of the coral skeleton.
Uses & applications
Primarily used as decorative lapidary material, collectors' specimens, and jewelry (especially when polished). They are also used as educational tools for paleontological study.
Geological facts
This specific honeycomb pattern identifies it as a colonial coral. Favosites corals are often nicknamed 'Honeycomb Corals.' They went extinct during the Permian-Triassic extinction event roughly 251 million years ago.
Field identification & locations
Identify in the field by the distinct 'wasp nest' or honeycomb cellular pattern. Commonly found in the Great Lakes region (USA), particularly Michigan, and in limestone quarries globally. Look for circular or hexagonal pits on the surface of limestone rocks near ancient seafloors.
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