sedimentary
Petoskey Stone
Hexagonaria percarinata (Fossilized Rugose Coral)
Hardness: 3.0-3.5 (Mohs scale); Color: Gray to beige, turning dark gray or black when wet; Luster: Matte when raw, vitreous to waxy when polished; Crystal Structure: Hexagonal honeycombed pattern of ancient coral polyps; Cleavage: None/Indistinct; Specific Gravity: 2.6-2.7.
- Hardness
- 3
- Color
- Gray to beige, turning dark gray or black when wet
- Luster
- Matte when raw, vitreous to waxy when polished
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Physical properties
Hardness: 3.0-3.5 (Mohs scale); Color: Gray to beige, turning dark gray or black when wet; Luster: Matte when raw, vitreous to waxy when polished; Crystal Structure: Hexagonal honeycombed pattern of ancient coral polyps; Cleavage: None/Indistinct; Specific Gravity: 2.6-2.7.
Formation & geological history
Formed during the Devonian Period (approximately 350-400 million years ago) from the fossilization of colonial rugose corals. The silica and calcium carbonate in the corals were replaced by calcite over millions of years through permineralization within sedimentary limestone layers.
Uses & applications
Primarily used in jewelry (cabochons, rings, pendants), ornamental carvings, and as a popular collector's item in the lapidary arts. It is also the official state stone of Michigan.
Geological facts
A Petoskey stone is unique because it is technically a fossil. Its distinctive name comes from Chief Ignatius Petosegay, an Odawa chief whose name meant 'Rising Sun.' When found on beaches, the stones often look like ordinary gray rocks until they are dipped in water, which reveals the 'sunray' coral pattern.
Field identification & locations
Commonly found along the shores of Lake Michigan and Lake Huron. In the field, look for a grayish rock with a honeycomb or multi-sided figure pattern; if the pattern is faint, wet the stone to make it stand out. Collectors should look for defined 'eyes' or centers within each hexagonal cell.
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