Rock Identifier
Fossil Coral (Scleractinia / Rugosa (Silicified Coral)) — fossil
fossil

Fossil Coral

Scleractinia / Rugosa (Silicified Coral)

Hardness: 6.5 to 7 on the Mohs scale (due to quartz replacement); Color: White, grey, tan, with brownish patches; Luster: Vitreous to dull; Crystal structure: Microcrystalline quartz (chalcedony/agate replacement of original calcium carbonate).

Hardness
6
Color
White, grey, tan, with brownish patches
Luster
Vitreous to dull
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Physical properties

Hardness: 6.5 to 7 on the Mohs scale (due to quartz replacement); Color: White, grey, tan, with brownish patches; Luster: Vitreous to dull; Crystal structure: Microcrystalline quartz (chalcedony/agate replacement of original calcium carbonate).

Formation & geological history

Formed in ancient shallow marine environments where coral skeletons were buried in sediment and subsequently replaced by silica-rich groundwater over millions of years (often Paleozoic to Cenozoic eras).

Uses & applications

Used in lapidary arts, wire-wrapped jewelry, decorative cabochons, and as a popular specimen for fossil and rock collectors.

Geological facts

Many fossil corals undergo a process called silicification, where the original aragonite or calcite of the coral skeleton is entirely replaced by quartz, preserving the delicate cellular and polyp structures in stone.

Field identification & locations

Easily identified in the field or riverbeds by its distinctive honeycomb or star-like patterned calices (corallites) on the surface; commonly found in areas with ancient limestone deposits or glacial drift.

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