How to identify Petoskey Stone

To correctly identify Petoskey Stone (Fossilized Hexagonaria 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 brown, sometimes with reddish or yellowish hues due to iron staining. The patterns are often lighter 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 porous when unpolished; smooth and often cool to the touch when polished. The hexagonal patterns are tactilely distinct on unpolished surfaces.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not a true crystal; it is a fossilized biological structure. The characteristic form is the hexagonal pattern of the corallites, typically 2-10 mm in diameter, with radiating septa and a central columella (mouth).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No true cleavage due to its fossilized, cryptocrystalline nature. It breaks irregularly.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in glacial till, gravel pits, and along the shores of lakes and rivers, particularly in areas where Devonian bedrock has been eroded by glaciers. Original formation environment was a shallow, warm marine shelf.

Key Facts

Hardness

3 on the Mohs scale (due to its 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 lighter hexagonal patterns.

Luster

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

Transparency

Opaque.

Fracture

Conchoidal to uneven.

Cleavage

Perfect rhombohedral (for individual calcite crystals, but not observable in the fossil aggregate).

Composition

Primarily calcium carbonate (CaCO3), often partially silicified (replaced by SiO2).

Physical characteristics

• Crystal Habit: Not a single crystal; it is a fossilized colonial organism. The internal structure of the fossil is composed of cryptocrystalline calcite.
• Cleavage Type: Not applicable to the overall fossil; constituent calcite has perfect rhombohedral cleavage.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Dull to earthy (unpolished), vitreous to waxy (polished).

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