Rock Identifier
Fossilized Coral (Hexagonaria (Rugose and Tabulate coral fossils)) — fossil
fossil

Fossilized Coral

Hexagonaria (Rugose and Tabulate coral fossils)

Hardness: 6.5 to 7 on the Mohs scale (due to silicification/chalcedony replacement); Color: Brown, tan, grey, or yellowish; Luster: Vitreous to waxy when polished; Crystal structure: Microcrystalline quartz (chalcedony/agate replacement) retaining original coral structure.

Hardness
6
Color
Brown, tan, grey, or yellowish
Luster
Vitreous to waxy when polished
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Physical properties

Hardness: 6.5 to 7 on the Mohs scale (due to silicification/chalcedony replacement); Color: Brown, tan, grey, or yellowish; Luster: Vitreous to waxy when polished; Crystal structure: Microcrystalline quartz (chalcedony/agate replacement) retaining original coral structure.

Formation & geological history

Formed during the Paleozoic era, specifically the Devonian period (around 350-400 million years ago), when warm shallow seas covered regions like Michigan and parts of the Midwest. Ancient coral skeletons were gradually replaced by silica-rich groundwaters over millions of years.

Uses & applications

Used primarily in lapidary arts, cabochons for jewelry making, decorative carvings, and as sought-after specimens for fossil and rock collectors.

Geological facts

Many fossilized coral specimens, such as Petoskey stone, become much more distinct and show their cellular coral patterns clearly when wet or polished.

Field identification & locations

Can be identified in the field by its characteristic hexagonal or web-like coral corallite patterns on the surface. Commonly found in gravel beds, glacial tills, and sedimentary limestone formations.

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