Rugose corals are extinct solitary or colonial organisms known for their distinctive horn-shaped exoskeleton, often referred to as horn coral. They are commonly found in Paleozoic limestone deposits, where collectors prize them for their clear internal septal arrangements visible in cross-section. Specimens are typically preserved through calcite replacement, often forming robust fossils that are easy to clean and display.
Is this rugose coral?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch rugose coral with a known reference. Rugose Coral sits at Mohs 3-4, softer than the next harder reference, harder than the previous one.
- 2Name the organismDecide what left the fossil before worrying about the rock. Organism: Rugose corals (class Rugosa). Age: Middle Ordovician to Late Permian.
- 3Check the preservationPreservation: Calcite skeleton. That controls the weight, the hardness and how the piece takes a polish.
- 4Read the lusterHold the specimen under a strong light. Luster: dull to vitreous.
- 5Match the color rangeCompare against the expected color range: white, gray, brown, tan.
Often found alongside rugose coral
Minerals reported to co-occur with rugose coral. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Mohs hardness
- 3-4
- Density
- 2.6-2.9 g/cm³
- Luster
- Dull to vitreous
- Transparency
- Opaque
- Rarity
- Common
- Uses
- Collector, Decorative, Paleontological study
- Host rock
- limestone
- Organism
- Rugose corals (class Rugosa)
- Age
- Middle Ordovician to Late Permian
- Preservation
- Calcite skeleton
Where rockhounds find rugose coral
Classic worldwide localities
- Indiana
- Kentucky
- Ohio
- Gotland
- Morocco
Field-hunting tip
Where it forms: limestone. If you start seeing calcite, dolomite, quartz in float, you are in the right ground.
Common questions
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Find rugose coral on the map
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