sedimentary
Fossil Coral (Petoskey Stone or similar)
Hexagonaria percarinata (Fossilized Rugose Coral)
Hardness: 3.0-4.0 (Mohs scale), Color: light grey to beige, Luster: earthy to vitreous when polished, Crystal structure: Hexagonal/tabular patterns of corallites, Composition: Calcite (CaCO3)
- Hardness
- 3
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Physical properties
Hardness: 3.0-4.0 (Mohs scale), Color: light grey to beige, Luster: earthy to vitreous when polished, Crystal structure: Hexagonal/tabular patterns of corallites, Composition: Calcite (CaCO3)
Formation & geological history
Formed during the Devonian Period (approx. 350-400 million years ago) from ancient saltwater reefs. Over time, the living coral was replaced by calcite and minerals through sediment deposition and lithification.
Uses & applications
Primarily used for lapidary work, jewelry (beads, pendants), and as a collector's specimen. Polished versions are popular gifts.
Geological facts
The Petoskey stone is the state stone of Michigan. The name comes from an Ottawa Chief, Pe-to-se-ga. These fossils represent a time when much of the northern US was covered by warm, shallow tropical seas.
Field identification & locations
Identify by looking for 'sunburst' or 'honeycomb' patterns on the surface, especially when wet. Found on beaches and in glacial till, particularly in the Great Lakes region of North America.
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