Rock Identifier
Fossilized Coral (Petoskey Stone or similar Rugose Coral) (Hexagonaria percarinata (or similar Rugose/Tabulate coral)) — fossil
fossil

Fossilized Coral (Petoskey Stone or similar Rugose Coral)

Hexagonaria percarinata (or similar Rugose/Tabulate coral)

Hardness: 3 to 4 on the Mohs scale (due to calcite composition); Color: Grey, brown, or tan with characteristic patterned honeycomb or cellular structure; Luster: Dull to vitreous when polished; Crystal structure: Trigonal (calcite); Cleavage: Perfect rhombohedral; Specific Gravity: 2.6 to 2.7.

Hardness
3 to 4 on the Mohs scale (due to calcite composition)
Luster
Dull to vitreous when polished
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Physical properties

Hardness: 3 to 4 on the Mohs scale (due to calcite composition); Color: Grey, brown, or tan with characteristic patterned honeycomb or cellular structure; Luster: Dull to vitreous when polished; Crystal structure: Trigonal (calcite); Cleavage: Perfect rhombohedral; Specific Gravity: 2.6 to 2.7.

Formation & geological history

Formed from ancient coral reefs during the Devonian period, approximately 350 to 400 million years ago, as coral skeletons were fossilized and replaced by minerals like calcite or silica in shallow inland seas.

Uses & applications

Popular in rock hounding, lapidary arts, making decorative items, jewelry, and as a collectible geological specimen.

Geological facts

When wet, the characteristic hexagonal patterns and coral structures become much more prominent, which is why beachcombers often search for these stones along lakeshores or in wet conditions.

Field identification & locations

Easily identifiable in the field by the distinctive pitted, patterned surface resembling honeycombs or clustered tubes. Commonly found in glacial drift, gravel pits, and shores of the Great Lakes region or limestone-rich areas.

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