Petoskey Stone

Description

The Petoskey Stone is a fossilized colonial coral, Hexagonaria percarinata, characterized by its distinctive hexagonal (six-sided) patterns, which are the cross-sections of the individual coral polyps (corallites). Each hexagon typically contains a small, dark central dot, which was the mouth of the coral polyp, surrounded by radiating septa (skeletal partitions). The color ranges from light gray to dark brown, often with a mottled appearance, and the patterns become most prominent when the stone is wet or polished.

How to Identify

Color

Typically gray to brown, sometimes with reddish or yellowish hues due to iron staining. The patterns are often lighter than the surrounding matrix.

Luster

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

Texture

Rough and porous when unpolished; smooth and often cool to the touch when polished. The hexagonal patterns are tactilely distinct on unpolished surfaces.

Crystal Form

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).

Cleavage

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

Geological Environment

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).

Quick Check

• Color: Gray to brown with lighter hexagonal patterns.
• Luster: Dull to earthy (unpolished), vitreous to waxy (polished).
• Streak: White (as it is primarily calcite).

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).

Formation

The Petoskey Stone is a fossilized rugose coral, specifically Hexagonaria percarinata. These corals lived in warm, shallow, marine environments during the Devonian Period. Upon death, their calcium carbonate skeletons were buried in marine sediments. Over millions of years, these sediments lithified into limestone, and the original aragonite of the coral skeleton was replaced by calcite through a process of diagenesis and silicification, preserving the distinctive hexagonal pattern of the coral polyps. Glacial activity during the Pleistocene Epoch subsequently eroded these Devonian limestones, transporting and depositing fragments across Michigan, particularly in the northern Lower Peninsula.

Usage

Primarily used as a decorative stone, for lapidary work (cabbing, carving, polishing), and as a collectible. It is the state stone of Michigan. It is often made into jewelry, paperweights, and other ornamental objects.

Age Distribution

Devonian Period (approximately 350 to 400 million years ago)

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Not a mineral, but a fossilized rock. The primary mineral component is Calcite.

Group

Fossilized Coral (Rugose Coral)

Crystal System

Trigonal (for calcite)

Chemical Formula

CaCO3 (for the primary mineral calcite, with potential SiO2 replacement)

Composition

Calcium carbonate (calcite) with varying degrees of silica replacement.

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