Rock Identifier
Fossil Coral (Rugosa or Tabulata / Hexacorallia (replaced with silica or calcite)) — fossil
fossil

Fossil Coral

Rugosa or Tabulata / Hexacorallia (replaced with silica or calcite)

Hardness varies from 3 to 7 depending on mineral replacement (usually quartz/chalcedony or calcite); colors range from white, grey, brown to yellowish; luster is vitreous to dull; crystal structure is cryptocrystalline when silicified; specific gravity is 2.6 to 2.7.

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Physical properties

Hardness varies from 3 to 7 depending on mineral replacement (usually quartz/chalcedony or calcite); colors range from white, grey, brown to yellowish; luster is vitreous to dull; crystal structure is cryptocrystalline when silicified; specific gravity is 2.6 to 2.7.

Formation & geological history

Formed in ancient shallow marine environments, typically millions of years old (Paleozoic to Cenozoic eras), where coral skeletons were preserved and subsequently replaced by minerals like silica or calcite through permineralization.

Uses & applications

Used primarily in lapidary arts for cabochons, jewelry, ornamental carvings, and as a popular specimen for fossil collectors.

Geological facts

Fossil coral often shows intricate honeycomb or star-like patterns formed by the original coral polyps. When the coral is replaced by agate or chalcedony, it is frequently referred to as 'agatized coral' or 'petoskey stone'.

Field identification & locations

Easily identified by the distinctive porous or cellular honeycomb patterns on the surface. Found worldwide in sedimentary rock formations that were once ancient oceans, such as in Florida, Michigan, and Utah.

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