Rock Identifier
Fossil Coral (Hexagonaria or Rugose coral fossil (replaced by silica/chalcedony)) — fossil
fossil

Fossil Coral

Hexagonaria or Rugose coral fossil (replaced by silica/chalcedony)

Hardness: 6.5 to 7 on the Mohs scale (due to silicification); Color: Tan, brown, grey, or white; Luster: Vitreous to dull; Crystal structure: Trigonal/Microcrystalline (quartz replacement); Cleavage: None

Hardness
6
Color
Tan, brown, grey, or white
Luster
Vitreous to dull
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Physical properties

Hardness: 6.5 to 7 on the Mohs scale (due to silicification); Color: Tan, brown, grey, or white; Luster: Vitreous to dull; Crystal structure: Trigonal/Microcrystalline (quartz replacement); Cleavage: None

Formation & geological history

Formed in warm, shallow ancient seas during the Paleozoic era (Devonian period, approximately 350-400 million years ago) when coral skeletons were gradually replaced by silica-rich groundwater over millions of years.

Uses & applications

Used primarily as a decorative stone, in lapidary arts for cabochons and jewelry, and as a prized specimen for fossil and mineral collectors.

Geological facts

Many fossil coral specimens, particularly those from the Great Lakes region of the United States like Petoskey stones, are state stones and show distinct hexagonal honeycomb structures representing ancient coral calyces.

Field identification & locations

Identified by its distinctive honeycomb or star-like pattern of cellular pores on the surface. Commonly found in sedimentary rock formations, glacial drifts, and ancient reef deposits worldwide.

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