How to identify Petoskey Stone

To correctly identify Petoskey Stone (Petoskey Stone (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 to brown, often appearing darker when wet or polished, revealing the pattern more clearly.

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.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Rough and porous when unpolished; smooth and often cool to the touch when polished.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not a true crystal in the mineralogical sense, but the fossilized structure exhibits hexagonal patterns (corallites) with radiating septa, which are the defining characteristic.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   As a limestone, it does not exhibit distinct mineral cleavage. Fracture is typically conchoidal to uneven.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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.

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).

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