fossil

Petoskey Stone

Hexagonaria percarinata (Fossilized Rugose Coral)

Hardness: 3.0-3.5 (Mohs scale); Color: Gray to light tan, darker pattern when wet; Luster: Dull to waxy when polished; Crystal Structure: Hexagonal patterns (corallites) with radiating lines (septa); Composition: Calcite (CaCO3).

Hardness: 3
Color: Gray to light tan, darker pattern when wet
Luster: Dull to waxy when polished

Identified July 19, 2026More fossil →

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Physical properties

Formation & geological history

Formed from ancient coral reefs during the Devonian Period (approximately 350-400 million years ago). The coral colony died and was eventually buried under sediment, fossilizing as the organic matter was replaced by calcite.

Uses & applications

Primarily used in jewelry, decorative items, lapidary art, and as a popular souvenir/collector's item. It is the official State Stone of Michigan.

Geological facts

The unique 'honeycomb' or 'six-sided star' pattern only appears clearly when the stone is wet or polished. The name 'Petoskey' comes from an Ottawa Indian Chief named Pet-O-Sega, meaning 'rising sun', 'rays of dawn', or 'sunbeams of promise'.

Field identification & locations

Identify by looking for the distinctive hexagonal cell patterns. Commonly found along the shores of Lake Michigan and Lake Huron, especially around Petoskey and Traverse City. In the field, look for gray rocks that show patterns when splashed with water.

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