fossil

Petoskey Stone

Hexagonaria percarinata

Mohs hardness of 3-4, brown to gray color when dry, vitreous to dull luster, hexagonal corallite structures, specific gravity around 2.6.

Identified November 13, 2026More fossil →

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Physical properties

Mohs hardness of 3-4, brown to gray color when dry, vitreous to dull luster, hexagonal corallite structures, specific gravity around 2.6.

Formed by the fossilization of colonial rugose corals during the Devonian period, approximately 350 million years ago, in ancient shallow seas.

Widely used as a decorative gemstone, polished for jewelry, and collected as Michigan's state stone.

The distinctive pattern resembles a sunburst of hexagons, which becomes much more visible when the stone is wet or polished.

Easily identified by its characteristic mosaic-like hexagonal coral pattern, found primarily in the glacial drift of northern Michigan.

Probable fossil-bearing rock or mineral concretion

Unidentified carbonate-rich concretion with a possible fossil inclusion

Likely fossil oyster shell

Fossilized bivalve shell; exact species cannot be identified from the image

Fossil

Invertebrate fossil / Trace fossil

Crinoid Stem Fossil in Matrix

Phylum Echinodermata, Class Crinoidea

Fossiliferous limestone (likely a crinoidal or shell-fragment limestone)

Bioclastic limestone, composed chiefly of calcite (CaCO₃)

Porous limestone pebble

Calcite-rich limestone (primarily CaCO₃); exact identification is uncertain from the image