fossil

Fossilized Coral

Rugose coral / Tabulate coral fossil in limestone matrix

Hardness: 3-4 on the Mohs scale (for calcite); Color: Tan, brown, grey, or white; Luster: Dull to vitreous; Crystal structure: Hexagonal/trigonal (calcite replacement); Cleavage: Perfect rhombohedral; Specific Gravity: 2.6-2.7

Hardness: 3-4 on the Mohs scale (for calcite)
Color: Tan, brown, grey, or white
Luster: Dull to vitreous

Identified August 3, 2026More fossil →

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Physical properties

Formation & geological history

Formed when ancient coral colonies in warm, shallow prehistoric seas (dating from the Paleozoic era, particularly Silurian to Carboniferous, roughly 300-400 million years ago) were buried under sediment. Over millions of years, the original skeletal material (aragonite) was replaced by silica or calcite through permineralization.

Uses & applications

Popular as a lapidary material for cabochons, polished display specimens, jewelry, and educational or fossil collecting.

Geological facts

Many fossilized corals, such as Petoskey stones, are glacial erratics found polished by ice sheets and wave action along lake shores. The distinctive honeycomb or web-like patterns are cross-sections of the coral corallites.

Field identification & locations

Can be identified in the field by looking for characteristic polygonal or star-like corallite patterns on weathered limestone or rounded pebbles. Commonly found in regions with extensive ancient marine sedimentary rocks, such as the Midwestern United States and parts of Europe.

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Fossilized Crinoid Stem in Limestone

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Fossiliferous Limestone

Limestone with embedded fossil cross-section

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Fossiliferous Limestone with Crinoid stems

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