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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 patterns, which are the fossilized corallites (individual coral polyps) with radiating septa. The color typically ranges from gray to tan to brown, often with lighter patterns against a darker background when wet or polished. It is a type of fossiliferous limestone, meaning it is a sedimentary rock composed largely of fossilized organic material.
How to Identify
- Color
- Typically gray to tan to brown, often appearing darker when wet or polished, revealing the pattern more clearly.
- Luster
- Dull to earthy in its natural, unpolished state; vitreous (glassy) to waxy when polished.
- Texture
- Rough and porous when unpolished; smooth and often cool to the touch when polished.
- Crystal Form
- Not a true crystal in the mineralogical sense, but the fossilized structure exhibits hexagonal patterns (corallites) with radiating septa, which are the defining characteristic.
- Cleavage
- As a limestone, it does not exhibit distinct mineral cleavage. Fracture is typically conchoidal to uneven.
- Geological Environment
- Found in ancient marine sedimentary deposits, specifically within the Traverse Group formations of the Devonian Period. These environments were warm, shallow seas conducive to coral reef growth.
Key Facts
- Hardness: 3 on the Mohs scale (due to calcite composition).
- Specific Gravity: Approximately 2.71 (typical for calcite/limestone).
- Crystal System: Trigonal (for the constituent calcite mineral). The overall fossil structure is not a single crystal.
- Color: Gray, tan, brown, often with lighter hexagonal patterns.
- Luster: Dull to earthy (unpolished), vitreous to waxy (polished).
- Transparency: Opaque.
- Fracture: Conchoidal to uneven.
- Cleavage: Perfect rhombohedral cleavage (of the constituent calcite), but not typically observed in the macroscopic rock specimen.
- Composition: Primarily calcium carbonate (CaCO3) in the form of calcite, which replaced the original aragonite of the coral skeleton.
Quick Check
- Color: Gray to tan/brown, often darker when wet.
- Luster: Dull to earthy (unpolished), vitreous to waxy (polished).
- Streak: White (due to calcite composition).
Physical Characteristics
- Crystal Habit: Not applicable for the rock as a whole; the constituent calcite forms microscopic rhombohedral crystals.
- Cleavage Type: Perfect rhombohedral (for calcite).
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Dull to earthy (unpolished), vitreous to waxy (polished).
Formation
Formed from the fossilized remains of the rugose coral Hexagonaria percarinata. These corals lived in warm, shallow, marine environments during the Devonian Period. After the corals died, their calcium carbonate skeletons were buried and underwent diagenesis, a process where the original aragonite of the coral skeleton was replaced by calcite, and the surrounding sediment lithified into limestone. The characteristic hexagonal pattern is the cross-section of the individual coral polyps (corallites) and their septa.
Usage
Primarily used as an ornamental stone for jewelry (cabochons, beads), carvings, polished display pieces, and decorative items. It is also collected by hobbyists and tourists.
Age Distribution
Devonian Period (approximately 359 to 419 million years ago)
Where to Find
Michigan, USA
Predominantly found along the shores of Lake Michigan and Lake Huron, particularly in the northern Lower Peninsula of Michigan, including Petoskey, Charlevoix, and other coastal areas. It is the state stone of Michigan.
Other Devonian outcrops
Similar fossilized rugose corals (Hexagonaria species) can be found in other Devonian-aged marine sedimentary rocks globally, but the specific Hexagonaria percarinata and its abundance leading to the 'Petoskey Stone' designation is unique to Michigan.
Finding Tips
Search along shorelines
The best places to find Petoskey Stones are along the beaches of Lake Michigan and Lake Huron, especially after storms or during periods of low water levels, as wave action exposes and polishes them.
Look for wet stones
The distinctive hexagonal pattern is much more visible when the stone is wet. Many collectors carry a small spray bottle of water to help identify them.
Examine gravel pits and road cuts
Sometimes found in gravel pits or road cuts that expose Devonian-aged sedimentary layers in northern Michigan.
Distinguish from other rocks
Be careful not to confuse them with other patterned rocks or chert. The hexagonal pattern is key.
Similar Rocks
Charlevoix Stone
Favosites (tabulate coral)
Also known as: Favosites
Honeycomb Coral
Favosites (tabulate coral)
Also known as: Favosites
Chain Coral
Halysites (tabulate coral)
Also known as: Halysites
Scientific Classification
- Mineral Class
- Carbonate (for calcite)
- Group
- Fossiliferous Sedimentary Rock
- Crystal System
- Trigonal (for calcite)
- Chemical Formula
- CaCO3 (for calcite)
- Composition
- Calcium carbonate (calcite) with minor impurities.
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