sedimentary
Orthoceras Fossil Limestone
Orthoceras (extinct Cephalopod fossil) in Matrix
Hardness: 3-4 (Mohs), Color: Grey/Black matrix with white/cream fossils, Luster: Vitreous when polished, Crystal structure: Microcrystalline calcite replacing organic matter, Specific gravity: 2.7
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Physical properties
Hardness: 3-4 (Mohs), Color: Grey/Black matrix with white/cream fossils, Luster: Vitreous when polished, Crystal structure: Microcrystalline calcite replacing organic matter, Specific gravity: 2.7
Formation & geological history
Formed in marine environments during the Paleozoic era, specifically the Ordovician period (approx. 480-440 million years ago). The straight-shelled nautiloids sank to the seafloor and were buried in lime mud.
Uses & applications
Primarily used for decorative items like spheres, bookends, tiles, and table tops. Also highly valued in fossil collecting and metaphysical communities.
Geological facts
The name 'Orthoceras' means 'Straight Horn'. These creatures were ancestors to modern squids and octopuses and could reach several feet in length. They used nitrogen gas in their chambers to control buoyancy.
Field identification & locations
Identify by the presence of conical, chambered shells with visible siphuncles (internal tubes). Most commercial specimens are sourced from the Anti-Atlas Mountains of Morocco.
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