Sedimentary/Fossil

Coral Fossil (Agatized Coral or Petoskey Stone)

Hexagonaria percarinata (if Petoskey) or generally Scleractinia (Agatized Coral)

AI-generated identification · may be incorrect

Hardness: 6.5-7 (if agatized) or 3 (if calcite), Color: Tan, brown, gray, with white or bluish spots, Luster: Waxy to vitreous (polished), Crystal structure: Microcrystalline quartz or calcite replacing original organic structure, Cleavage: None

Hardness: 6

Identified June 19, 2026More sedimentary/fossil →

Explore Coral Fossil (Agatized Coral or Petoskey Stone) in the encyclopedia →

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Physical properties

Formation & geological history

Formed in ancient marine environments where coral reefs thrived. Over millions of years, the organic material was replaced by silica (agatization) or calcite. Geological age: Often Devonian (around 350 million years ago) or younger depending on the specific type.

Uses & applications

Used in lapidary arts, jewelry making, and collected as specimens.

Geological facts

Fossil corals are often state stones, like the Petoskey stone (state stone of Michigan). The circular patterns are the cross-sections of the individual coral polyps.

Field identification & locations

Identify by the distinct coral polyp patterns (circles or hexagons) within the stone. Found in areas that were once ancient shallow seas, such as Michigan (US), Indonesia, or Florida (US). Tip: Agatized coral is harder than calcite fossil coral.