fossil

Fossilized Coral

Rugosa / Tabulata coral fossils replaced by silica (Chalcedony/Quartz)

AI-generated identification · may be incorrect

Hardness: 6.5 to 7 on the Mohs scale (due to silicification). Color: Varies widely including tan, brown, gray, and white. Luster: Vitreous to dull. Crystal structure: Trigonal (microcrystalline quartz). Cleavage: None. Specific gravity: 2.58 to 2.66.

Hardness: 6
Color: Varies widely including tan, brown, gray, and white
Luster: Vitreous to dull

Identified August 3, 2026More fossil →

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Physical properties

Formation & geological history

Formed through the process of permineralization or replacement, where ancient coral skeletons (dating from the Paleozoic era, hundreds of millions of years ago) were gradually replaced by mineral-rich groundwater carrying silica, preserving the intricate internal structure.

Uses & applications

Used primarily in lapidary work, cabochons for jewelry, decorative display pieces, and by fossil and mineral collectors.

Geological facts

Fossil coral is often the state stone of states with ancient marine history, such as Michigan (Petoskey stone). The distinctive cellular patterns reveal the growth structures of ancient marine invertebrates that lived in shallow tropical seas.

Field identification & locations

Can be identified in the field by characteristic honeycomb or star-like patterns on the weathered surface. Commonly found in sedimentary limestone formations and glacial drift deposits in regions previously covered by ancient oceans.