sedimentary
Petoskey Stone
Hexagonaria percarinata (Fossilized Rugose Coral)
Hardness: 3-4 (Mohs scale), Color: Gray to tan when dry (features visible when wet), Luster: Dull to earthy (glossy when polished), Structure: Massive colonial coral with distinct hexagonal fossil patterns, Specific Gravity: 2.7
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Physical properties
Hardness: 3-4 (Mohs scale), Color: Gray to tan when dry (features visible when wet), Luster: Dull to earthy (glossy when polished), Structure: Massive colonial coral with distinct hexagonal fossil patterns, Specific Gravity: 2.7
Formation & geological history
Formed through the fossilization of colonial rugose corals in warm, shallow seas. These date back to the Devonian Period, approximately 350 million years ago, when Michigan was near the equator.
Uses & applications
Popular for lapidary work, jewelry (pendants, rings), decorative ornaments, and as a highly sought-after collector's item in the Great Lakes region.
Geological facts
It is the official state stone of Michigan. The name 'Petoskey' reportedly means 'Rays of the Rising Sun' in the Odawa language, referring to the pattern of the fossil.
Field identification & locations
Identify by wetting the stone to reveal the 'honeycomb' or 'six-sided star' pattern. Commonly found on beaches along the northern shores of Michigan's lower peninsula, particularly after storms.
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