sedimentary
Petoskey Stone
Hexagonaria percarinata (Fossilized Rugose Coral)
Hardness: 3-4 (Mohs), Color: Gray/brown when dry; patterned when wet or polished, Luster: Vitreous to dull, Structure: Hexagonal colonial cells with radiating septa, Specific Gravity: ~2.7
- Hardness
- 3-4 (Mohs), Color: Gray/brown when dry
- Color
- Gray/brown when dry
Identified More sedimentary →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 3-4 (Mohs), Color: Gray/brown when dry; patterned when wet or polished, Luster: Vitreous to dull, Structure: Hexagonal colonial cells with radiating septa, Specific Gravity: ~2.7
Formation & geological history
Formed during the Devonian Period about 350 million years ago. It is fossilized coral (Hexagonaria) that was altered as silica-rich groundwater replaced the calcium carbonate structure during the rock-forming process.
Uses & applications
Used primarily as decorative carvings, jewelry, lapidary art, and souvenirs. It is highly valued as a collectible and is the official state stone of Michigan.
Geological facts
A Petoskey stone is actually a fossil of a colonial coral that lived in warm, shallow seas. When dry, it looks like ordinary limestone, but when wet or polished, the distinctive 'six-sided' star pattern of the coral skeletons appears.
Field identification & locations
Identify in the field by looking for a 'honeycomb' or star-like pattern on the surface of gray stones, especially along shorelines of Lake Michigan and Lake Huron. It often requires being wet to see the pattern clearly.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Sandstone
Arenite (primarily SiO2)
sedimentary
Chert Breccia
Brecciated Chert (Microcrystalline Silica)
sedimentary
Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
sedimentary
Shale or Slaty Mudstone
Argillaceous sedimentary rock
sedimentary
Sandstone
Arenite
sedimentary