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The Petoskey Stone is a fossilized coral, specifically a rugose coral of the species Hexagonaria percarinata. It is characterized by its distinctive hexagonal, often polygonal, patterns which are the cross-sections of the individual coral polyps (corallites). The color typically ranges from gray to tan to brown, often with lighter colored septa (the radial walls within each corallite) that create the characteristic 'eye' pattern. When wet or polished, the patterns become much more pronounced and visually striking. It is the state stone of Michigan, USA.
How to Identify
- Color
- Typically gray to tan to brown, sometimes with reddish or yellowish hues due to iron staining. The patterns are often lighter in color.
- Luster
- Dull to earthy when unpolished; vitreous to waxy when polished.
- Texture
- Rough and granular when unpolished; smooth and often waxy when polished. The surface reveals the distinct hexagonal patterns.
- Crystal Form
- Not a true crystal in the mineralogical sense, but rather a fossilized organic structure. The 'form' is the preserved colonial coral structure with individual hexagonal corallites.
- Cleavage
- None, as it is a cryptocrystalline material (chert/chalcedony) replacing an organic structure.
- Geological Environment
- Found in glacial till, lakebeds, and exposed bedrock formations that were once ancient Devonian seafloors. These environments were warm, shallow, tropical seas conducive to coral growth.
Key Facts
- Hardness: 6.5-7 on the Mohs scale (due to its chert/chalcedony composition).
- Specific Gravity: 2.58-2.64 (typical for chert/chalcedony).
- Crystal System: Cryptocrystalline (no macroscopic crystal structure, composed of microscopic quartz crystals).
- Color: Gray, tan, brown, often with lighter patterns.
- Luster: Dull to earthy (unpolished), vitreous to waxy (polished).
- Transparency: Opaque.
- Fracture: Conchoidal to uneven.
- Cleavage: None.
- Composition: Silica (SiO2), primarily chert or chalcedony, which has replaced the original calcium carbonate (CaCO3) of the coral skeleton.
Quick Check
- Color: Gray to tan with lighter hexagonal patterns.
- Luster: Dull to earthy (unpolished), vitreous to waxy (polished).
- Streak: White (due to the chert/chalcedony composition).
Physical Characteristics
- Crystal Habit: Not applicable; fossilized organic structure replaced by cryptocrystalline silica.
- Cleavage Type: None.
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Dull to earthy (unpolished), vitreous to waxy (polished).
Formation
Formed from the fossilization of colonial rugose corals (Hexagonaria percarinata) that lived in warm, shallow, marine environments during the Devonian Period. The original calcium carbonate skeleton of the coral was replaced by silica (chert/chalcedony) through a process of permineralization and replacement, preserving the distinctive hexagonal pattern of the coral polyps.
Usage
Primarily used as an ornamental stone, for lapidary work (cabbing, polishing), jewelry, carvings, and decorative objects. It is also a popular collectible due to its unique appearance and geological significance.
Age Distribution
Devonian Period (approximately 350-400 million years ago)
Where to Find
Michigan, USA
Primarily found in the northern Lower Peninsula of Michigan, particularly along the shores of Lake Michigan and Lake Huron. Key locations include Petoskey, Charlevoix, Traverse City, and other beaches and gravel pits in the region. It is the state stone of Michigan.
Other Devonian Outcrops
Similar fossilized corals (though not specifically Hexagonaria percarinata or 'Petoskey Stone' by common name) can be found in other regions with Devonian-aged marine sedimentary rocks, such as parts of New York, Indiana, and Ohio in the USA, and various locations globally.
Finding Tips
Search on Beaches
The best time to find Petoskey Stones is after a storm or during early spring when ice melt and wave action have churned up the lake bottom and deposited new stones on the shorelines of Lake Michigan and Lake Huron.
Look for Wet Stones
The distinctive hexagonal pattern is much more visible when the stone is wet. Carry a small spray bottle of water or look for stones near the waterline.
Examine Gravel Beds
Gravel pits and road cuts in northern Michigan can also yield Petoskey Stones, as these often expose glacial till containing the fossils.
Distinguish from Other Rocks
Look for the characteristic 'honeycomb' or 'turtle shell' pattern. Other common beach rocks like limestone or granite will not have this distinct fossil structure.
Similar Rocks
Charlevoix Stone
Favosites spp.
Also known as: Favosites
Honeycomb Coral
Favosites spp.
Also known as: Favosites
Chain Coral
Halysites spp.
Also known as: Halysites
Scientific Classification
- Mineral Class
- Not a mineral, but a fossilized organism composed of cryptocrystalline quartz (chert/chalcedony).
- Group
- Fossilized Coral (Rugose Coral)
- Crystal System
- Cryptocrystalline (composed of microscopic quartz crystals).
- Chemical Formula
- SiO2 (for the replacing material, chert/chalcedony).
- Composition
- Silica (Silicon Dioxide) replacing original calcium carbonate.
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