sedimentary
Petoskey Stone (Petrified Favosites)
Favosites (extinct colonial coral), CaCO3 with Silica replacement
Hardness: 3-4 on Mohs scale (can be higher if silicated); Color: Brown, tan, or gray; Luster: Dull to earthy (glassy when polished); Structure: Hexagonal corallites forming a honeycomb pattern.
- Hardness
- 3-4 on Mohs scale (can be higher if silicated)
- Color
- Brown, tan, or gray
- Luster
- Dull to earthy (glassy when polished)
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Physical properties
Hardness: 3-4 on Mohs scale (can be higher if silicated); Color: Brown, tan, or gray; Luster: Dull to earthy (glassy when polished); Structure: Hexagonal corallites forming a honeycomb pattern.
Formation & geological history
Formed during the Devonian Period (approx. 350-400 million years ago) from the remains of ancient coral reefs in warm, shallow seas. Over millions of years, the coral was buried and mineralized.
Uses & applications
Used primarily for lapidary work, jewelry (pendants and beads), and as a popular collector's item or souvenir, particularly in the Great Lakes region.
Geological facts
It is the state stone of Michigan. The unique 'eye' pattern is only visible when the stone is wet or polished; when dry, it often looks like an ordinary gray rock.
Field identification & locations
Identify by looking for the distinct 'honeycomb' or 'hexagon' pattern on the surface. Commonly found on beaches and in gravel pits across the Lower Peninsula of Michigan, particularly near Petoskey and Charlevoix.
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