How to identify Petoskey Stone

To correctly identify Petoskey Stone (Fossilized Hexagonaria percarinata coral in limestone), 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 whitish. The fossil pattern is often darker than the surrounding matrix.

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 slightly porous when polished. The hexagonal patterns are tactile.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not a true crystal; it is a fossilized colonial coral. The individual corallites exhibit a hexagonal cross-section.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No distinct cleavage due to its sedimentary and fossiliferous nature. It will fracture irregularly.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in glacial till, lake shores, and riverbeds, primarily derived from Devonian-age bedrock formations that were exposed and eroded by glacial action.

Key Facts

Hardness

3-4 on the Mohs scale (due to its calcite composition).

Specific Gravity

Approximately 2.71 g/cm³ (typical for limestone).

Crystal System

Not applicable as it is a fossiliferous sedimentary rock; the primary mineral (calcite) is trigonal.

Color

Gray, tan, brown, white.

Luster

Dull to vitreous.

Transparency

Opaque.

Fracture

Uneven to conchoidal.

Cleavage

None (as a rock); calcite component has perfect rhombohedral cleavage.

Composition

Primarily calcium carbonate (CaCO₃) in the form of calcite, with fossilized Hexagonaria percarinata coral skeletons.

Physical characteristics

• Crystal Habit: Not applicable for the rock itself; the fossilized coral exhibits colonial, massive growth with hexagonal corallites.
• Cleavage Type: None for the rock; the constituent calcite has perfect rhombohedral cleavage.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Dull to earthy on unpolished surfaces; vitreous to waxy on polished surfaces.

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