fossil

Fossil Coral

Hexagonaria or Rugose coral fossil in limestone/silica matrix

AI-generated identification · may be incorrect

Mohs hardness: 3 to 4 (calcite) or up to 7 if silicified (quartz); Color: beige, gray, white, or brownish; Luster: dull to vitreous; Structure: hexagonal or tubular corallite patterns visible on the surface; Specific gravity: 2.6 to 2.7

Hardness: 3 to 4 (calcite) or up to 7 if silicified (quartz)
Color: beige, gray, white, or brownish
Luster: dull to vitreous

Identified August 2, 2026More fossil →

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Physical properties

Formation & geological history

Formed in ancient warm, shallow tropical marine environments, typically during the Paleozoic era (such as the Devonian period, approx. 350-400 million years ago), where coral skeletons were buried in sediment and mineralized or replaced by silica and calcite over millions of years.

Uses & applications

Used primarily in lapidary work, decorative cabochons for jewelry, ornamental display pieces, and as educational geological specimens for fossil collectors.

Geological facts

Fossil coral is the state stone of Michigan (specifically Petoskey stone, a fossilized rugose coral). It provides a direct window into ancient marine ecosystems that existed long before dinosaurs.

Field identification & locations

Can be identified in the field by characteristic honeycomb, star-like, or tubular patterns on weathered stone surfaces. Often found in limestone-rich regions, riverbeds, and glacial drift deposits worldwide.