sedimentary
Fossiliferous Limestone with Rugose Coral
Calcite (CaCO3) containing a Rugosa (Horn Coral) fossil
Hardness: 3 (Mohs scale); Color: Cream to light grey; Luster: Dull/Earthy; Crystal structure: Hexagonal (calcite); Specific gravity: 2.71
- Hardness
- 3 (Mohs scale)
- Color
- Cream to light grey
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3 (Mohs scale); Color: Cream to light grey; Luster: Dull/Earthy; Crystal structure: Hexagonal (calcite); Specific gravity: 2.71
Formation & geological history
Formed in warm, shallow marine environments where calcium carbonate precipitated from seawater or accumulated from skeletal remains during the Paleozoic era (approx. 488–251 million years ago).
Uses & applications
Primarily used in construction (aggregate), cement manufacturing, and as a biological/geological specimen for educational and collecting purposes.
Geological facts
Rugose corals are often called 'Horn Corals' because of their distinctive horn-like shape. They were major reef-builders before becoming extinct at the end of the Permian period.
Field identification & locations
Identify by searching for ridged, conical, or segmented biological structures within a matrix that fizzes when exposed to weak acid (diluted HCl or vinegar). Commonly found in limestone outcrops in the Midwest USA and UK.
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