How to identify Petoskey Stone

To correctly identify Petoskey Stone (Fossilized Hexagonaria percarinata coral (calcite)), 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, with darker hexagonal patterns. The contrast becomes more pronounced 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) when polished.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Rough and porous 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 in the mineralogical sense, but rather a fossilized biological structure. The hexagonal patterns are the key identifying feature.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   As a fossilized coral composed of calcite, it exhibits the characteristic rhombohedral cleavage of calcite, though this is rarely observed in the macroscopic fossil structure.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in glacial till, gravel pits, and along the shores of lakes (especially Lake Michigan and Lake Huron) in Michigan, derived from Devonian-age limestones.

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 mineral). The fossil itself is amorphous in terms of crystal system.

Color

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

Luster

Dull to earthy (unpolished), Vitreous (polished).

Transparency

Opaque.

Fracture

Conchoidal to uneven (typical for calcite).

Cleavage

Perfect rhombohedral (for the constituent calcite mineral).

Composition

Calcium carbonate (CaCO3), primarily calcite.

Physical characteristics

• Crystal Habit: Not applicable for the fossil itself; the constituent calcite forms rhombohedral crystals, but these are microscopic within the fossil.
• Cleavage Type: Perfect rhombohedral (for calcite).
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Dull to earthy (unpolished), Vitreous (polished).

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