sedimentary
Petoskey Stone
Hexagonaria percarinata (Fossilized Rugose Coral)
Hardness: 3.5 (Mohs); Color: Grey to tan (brownish when wet); Luster: Matte when raw, vitreous when polished; Crystal Structure: Hexagonal fossil shapes; Cleavage: None (fossilized structure); Specific Gravity: 2.7
- Hardness
- 3
- Color
- Grey to tan (brownish when wet)
- Luster
- Matte when raw, vitreous when polished
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Physical properties
Hardness: 3.5 (Mohs); Color: Grey to tan (brownish when wet); Luster: Matte when raw, vitreous when polished; Crystal Structure: Hexagonal fossil shapes; Cleavage: None (fossilized structure); Specific Gravity: 2.7
Formation & geological history
Formed from the fossilization of colonial coral during the Devonian Period (approximately 350 million years ago) in shallow, warm tropical seas.
Uses & applications
Primarily used for jewelry, decorative carvings, souvenirs, and architectural accents. It is highly prized by rockhounds and collectors.
Geological facts
It is the state stone of Michigan. The unique 'eye' pattern is only visible when the stone is wet or polished because of the way the coral structures trap water or accept a finish.
Field identification & locations
Identify in the field by looking for a distinctive hexagonal pattern on grey pebbles, especially near the substrate of Lake Michigan beaches. Best found after a storm or in the spring when ice has moved stones to the shore.
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