How to identify Petoskey Stone

To correctly identify Petoskey Stone (Fossilized Hexagonaria percarinata), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Typically gray to tan, sometimes brownish or reddish, with darker 'eyes' (centers of the hexagons). The contrast is enhanced when wet or polished.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy in its natural, unpolished state; vitreous (glassy) to waxy when polished due to the silicification.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Rough and porous in its natural state; smooth and often cool to the touch when polished.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as it is a fossilized organic structure replaced by cryptocrystalline quartz. It fractures conchoidally.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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).

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).

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