How to identify Petoskey Stone

To correctly identify Petoskey Stone (Hexagonaria percarinata (fossilized coral)), 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 to brown, sometimes with reddish or yellowish hues due to iron staining. The patterns are often lighter in color.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy when unpolished; vitreous to waxy when polished.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Rough and granular when unpolished; smooth and often waxy when polished. The surface reveals the distinct hexagonal patterns.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as it is a cryptocrystalline material (chert/chalcedony) replacing an organic structure.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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.

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).

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