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Fossilized Hexagonaria percarinata, commonly known as Petoskey Stone, is a fossilized colonial rugose coral characterized by its distinctive hexagonal, honeycomb-like pattern. Each hexagon represents the cross-section of an individual coral polyp (corallite), with radiating septa visible within. The color typically ranges from gray to tan, often with darker 'eyes' or centers within each hexagon, which are the fossilized mouths of the polyps. The pattern becomes most apparent when the stone is wet or polished.
How to Identify
- Color
- Typically gray to tan, sometimes brownish or reddish, with darker 'eyes' (centers of the hexagons). The contrast is enhanced when wet or polished.
- Luster
- Dull to earthy in its natural, unpolished state; vitreous (glassy) to waxy when polished due to the silicification.
- Texture
- Rough and porous in its natural state; smooth and often cool to the touch when polished.
- Crystal Form
- Not a true mineral crystal form, but rather a pseudomorph after the original coral skeleton. The characteristic pattern is hexagonal (polygonal) corallites with radiating septa.
- Cleavage
- None, as it is a fossilized organic structure replaced by cryptocrystalline quartz. It fractures conchoidally.
- Geological Environment
- Found in marine sedimentary environments, specifically in limestone formations that were once ancient shallow, warm seas. The original corals thrived in these environments during the Devonian Period.
Key Facts
- Hardness: 6.5-7 on the Mohs scale (due to silicification).
- Specific Gravity: 2.60-2.65 (typical for quartz/chalcedony).
- Crystal System: Trigonal (for the constituent quartz/chalcedony, but the overall fossil structure is amorphous/cryptocrystalline).
- Color: Gray, tan, brown, sometimes reddish; patterns are darker.
- Luster: Dull to earthy (unpolished), vitreous to waxy (polished).
- Transparency: Opaque to translucent (thin sections).
- Fracture: Conchoidal to uneven.
- Cleavage: None.
- Composition: Primarily silicon dioxide (SiO2), a pseudomorph after calcium carbonate (CaCO3).
Quick Check
- Color: Gray to tan with darker hexagonal patterns.
- Luster: Dull to earthy (unpolished), vitreous to waxy (polished).
- Streak: White (due to the quartz composition).
Physical Characteristics
- Crystal Habit: Cryptocrystalline aggregate, pseudomorph after colonial coral structure.
- Cleavage Type: None.
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Dull to earthy (unpolished), vitreous to waxy (polished).
Formation
Fossilized Hexagonaria percarinata forms when the original calcium carbonate (aragonite) skeletal structures of the colonial rugose coral Hexagonaria percarinata are replaced by silica (quartz) through a process called permineralization or silicification. This occurs after the coral dies and is buried in marine sediments. Over geological time, groundwater rich in dissolved silica infiltrates the porous coral skeleton, and the silica precipitates, replacing the original carbonate material molecule by molecule, preserving the intricate internal and external structures of the coral polyps. The surrounding matrix is typically limestone or shale.
Usage
Primarily used as an ornamental stone for lapidary purposes, including cabochons, spheres, carvings, and polished display pieces. It is also popular in jewelry and as a collectible fossil. Due to its distinctive pattern, it is a popular souvenir from Michigan.
Age Distribution
Devonian Period (approximately 350 to 400 million years ago)
Where to Find
Michigan, USA
Predominantly found in the Traverse Group formations of the northern Lower Peninsula of Michigan, particularly along the shores of Lake Michigan and Lake Huron, and in riverbeds and gravel pits. Notable locations include Petoskey, Charlevoix, and other areas around Little Traverse Bay.
Indiana, USA
Less common, but similar Devonian coral fossils can be found in parts of Indiana.
Iowa, USA
Some Devonian coral occurrences are noted in Iowa, though Hexagonaria percarinata specifically is less prevalent than in Michigan.
Finding Tips
Search in Water
The distinctive pattern of Petoskey Stones is much more visible when the stone is wet. Search along shorelines, especially after storms, or in shallow water.
Look for Hexagonal Patterns
Identify the characteristic honeycomb-like hexagonal patterns on the surface of the stones. Each hexagon should have a small 'eye' or dot in the center.
Check Hardness
While the original coral was calcium carbonate (Mohs 3), the silicified fossil is much harder (Mohs 6.5-7), similar to quartz. This can help differentiate it from non-silicified coral fossils.
Visit Known Locations
Focus your search on well-known areas in Michigan, such as the beaches around Petoskey and Charlevoix, where they are abundant.
Similar Rocks
Charlevoix Stone
Favosites spp.
Also known as: Favosites Coral
Chain Coral
Halysites spp.
Also known as: Halysites Coral
Horn Coral
Rugosa spp.
Also known as: Rugose Coral (solitary)
Scientific Classification
- Mineral Class
- Silicate (specifically, a cryptocrystalline variety of quartz/chalcedony)
- Group
- Fossil (specifically, a silicified rugose coral)
- Crystal System
- Trigonal (referring to the constituent quartz, not the fossil structure)
- Chemical Formula
- SiO2 (pseudomorph after CaCO3)
- Composition
- Silicon dioxide, with trace impurities that impart color.
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