fossil

Tumbled fossil coral (often sold as “petoskey stone” or fossil coral)

Fossilized colonial coral; commonly Hexagonaria percarinata in true Petoskey stones, though this specimen’s pattern may represent another fossil coral

AI-generated identification · may be incorrect

Polished, dark gray to black limestone-like matrix with cream reticulate corallite patterns; calcite-rich, Mohs hardness about 3–4, dull to waxy luster, and no visible cleavage in the polished form. The repeated coral chambers are the key diagnostic feature.

Identified September 15, 2026More fossil →

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At a glance

Common name: Tumbled fossil coral (often sold as “petoskey stone” or fossil coral)
Scientific name: Fossilized colonial coral; commonly Hexagonaria percarinata in true Petoskey stones, though this specimen’s pattern may represent another fossil coral
Category: fossil
Identified: September 15, 2026
Method: Standard

AI Estimated Value

About $5–$25 for a common polished palm stone; $30–$100+ for larger, sharply patterned, well-polished specimens.

Not an official valuation

AI estimate for general interest only — not an appraisal. Actual value depends on condition, provenance, and market demand. Verify with a professional before buying, selling, or insuring.

This identification was generated by AI and may contain errors. Verify important details before relying on them — and never use an AI result to judge whether a specimen is safe to handle. Read the full disclaimer

Physical properties

Formation & geological history

A Devonian marine colonial coral was buried, mineralized, and preserved in carbonate sediment roughly 350–420 million years ago; later weathering liberated and rounding produced the tumbled shape.

Uses & applications

Used as a lapidary ornament, paperweight, cabochon, and jewelry stone; valued mainly for its fossil pattern and polish rather than gemstone rarity.

Geological facts

True Petoskey stone is Michigan’s state stone and displays six-sided corallites in a honeycomb pattern. Similar fossil corals occur worldwide and may have circular or irregular “eye” patterns.

Field identification & locations

Identify it by examining the surface for repeated individual corallites, often with central pits or radial septa; vinegar or dilute acid may fizz because of calcite, but testing can damage the polish. Common sources include Michigan, Indonesia, Morocco, and Madagascar.