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The Petoskey Stone is a fossilized coral, specifically Hexagonaria percarinata, embedded within a limestone matrix. Its distinctive pattern consists of tightly packed, six-sided (hexagonal) corallites, each with a dark central 'eye' and radiating septa. The color typically ranges from gray to tan, often with shades of brown or white, and the pattern becomes most prominent when wet or polished. It is a relatively soft stone, making it easy to work with for lapidary purposes.
How to Identify
- Color
- Typically gray to tan, sometimes brownish or whitish. The fossil pattern is often darker than the surrounding matrix.
- Luster
- Dull to earthy when unpolished; vitreous to waxy when polished.
- Texture
- Rough and granular when unpolished; smooth and often slightly porous when polished. The hexagonal patterns are tactile.
- Crystal Form
- Not a true crystal; it is a fossilized colonial coral. The individual corallites exhibit a hexagonal cross-section.
- Cleavage
- No distinct cleavage due to its sedimentary and fossiliferous nature. It will fracture irregularly.
- Geological Environment
- Found in glacial till, lake shores, and riverbeds, primarily derived from Devonian-age bedrock formations that were exposed and eroded by glacial action.
Key Facts
- Hardness: 3-4 on the Mohs scale (due to its calcite composition).
- Specific Gravity: Approximately 2.71 g/cm³ (typical for limestone).
- Crystal System: Not applicable as it is a fossiliferous sedimentary rock; the primary mineral (calcite) is trigonal.
- Color: Gray, tan, brown, white.
- Luster: Dull to vitreous.
- Transparency: Opaque.
- Fracture: Uneven to conchoidal.
- Cleavage: None (as a rock); calcite component has perfect rhombohedral cleavage.
- Composition: Primarily calcium carbonate (CaCO₃) in the form of calcite, with fossilized Hexagonaria percarinata coral skeletons.
Quick Check
- Color: Gray to tan with darker hexagonal patterns.
- Luster: Dull to earthy (unpolished), vitreous to waxy (polished).
- Streak: White (due to its limestone composition).
Physical Characteristics
- Crystal Habit: Not applicable for the rock itself; the fossilized coral exhibits colonial, massive growth with hexagonal corallites.
- Cleavage Type: None for the rock; the constituent calcite has perfect rhombohedral cleavage.
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Dull to earthy on unpolished surfaces; vitreous to waxy on polished surfaces.
Formation
Formed from the fossilized remains of the colonial rugose coral Hexagonaria percarinata, which lived in warm, shallow, marine environments during the Devonian Period. After the corals died, their skeletons were buried and lithified into limestone. Subsequent glacial activity during the Pleistocene Epoch eroded and transported these limestones, depositing them in the Great Lakes region, particularly around Lake Michigan.
Usage
Primarily used as a lapidary material for making polished stones, cabochons, jewelry, and decorative objects. It is also a popular collectible due to its unique fossil pattern and historical significance as Michigan's state stone.
Age Distribution
Devonian Period (approximately 350-400 million years ago)
Where to Find
Lake Michigan Shorelines (Michigan, USA)
Abundant along the shores of Lake Michigan, particularly in the northern Lower Peninsula of Michigan, including Petoskey, Charlevoix, and Traverse City areas. They are often found washed up on beaches.
Gravel Pits and Quarries (Michigan, USA)
Can be found in gravel pits and quarries that extract material from glacial deposits or Devonian bedrock in Michigan.
Riverbeds and Stream Beds (Michigan, USA)
Occasionally found in riverbeds and stream beds that drain areas with glacial deposits originating from Devonian bedrock.
Finding Tips
Look for the distinctive pattern
The hexagonal pattern with a central 'eye' is the key identifier. This pattern is most visible when the stone is wet, so searching along the waterline or wetting potential finds is recommended.
Search after storms or high waves
Storms and high wave action can churn up the lake bottom and deposit new stones on the beaches, increasing the chances of finding them.
Check gravel and cobble beaches
Petoskey Stones are typically found among other gravel and cobbles, not in sandy areas. Focus your search on rocky beaches.
Carry a small spray bottle or bucket
Wetting the stones will immediately reveal the characteristic pattern, making identification much easier.
Similar Rocks
Charlevoix Stone
Fossilized Favosites coral in limestone
Also known as: Favosites coral fossil
Alpena Stone
Fossilized various tabulate corals in limestone
Also known as: Tabulate coral fossil
Horn Coral
Fossilized solitary rugose corals
Also known as: Rugose coral fossil
Scientific Classification
- Mineral Class
- Not a mineral, but a rock. The primary mineral component is Calcite (Carbonates).
- Group
- Sedimentary Rock (Fossiliferous Limestone)
- Crystal System
- Not applicable for the rock; the primary mineral calcite is trigonal.
- Chemical Formula
- Primarily CaCO₃ (calcium carbonate) from the coral skeletons and limestone matrix.
- Composition
- Fossilized colonial rugose coral (Hexagonaria percarinata) embedded in a matrix of calcium carbonate (limestone).
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