How to identify Petoskey Stone

To correctly identify Petoskey Stone (Fossilized Hexagonaria percarinata (Rugose 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 light to dark gray, tan, or brown. The patterns are often darker than the surrounding matrix.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy when unpolished; vitreous (glassy) to waxy when polished.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

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

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not a true crystal; it is a fossilized skeletal structure. The characteristic form is the hexagonal pattern of the individual corallites, typically 3-10 mm in diameter, with radiating septa and a central 'eye'.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as it is a fossilized biological structure composed of microcrystalline calcite.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in glacial till, lakebeds, and along shorelines, primarily derived from Devonian-age limestones of the Traverse Group.

Key Facts

Hardness

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

Specific Gravity

2.71 (typical for calcite).

Crystal System

Trigonal (for the constituent calcite, but the overall fossil structure is amorphous/polycrystalline).

Color

Gray, tan, brown, often with darker hexagonal patterns.

Luster

Dull to earthy (unpolished), vitreous to waxy (polished).

Transparency

Opaque.

Fracture

Conchoidal to uneven (typical for calcite).

Cleavage

Perfect rhombohedral (for the constituent calcite, but not typically observed in the fossil itself due to its microcrystalline nature).

Composition

Calcium carbonate (CaCO3), primarily in the form of calcite.

Physical characteristics

• Crystal Habit: Not a single crystal; it is a fossilized colonial organism. The internal structure is microcrystalline calcite, preserving the original coral morphology.
• Cleavage Type: Not applicable to the fossil as a whole; constituent calcite exhibits perfect rhombohedral cleavage.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Dull to earthy (unpolished), vitreous to waxy (polished).

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