sedimentary

Orthoceras Fossil in Limestone

Orthoceras (extinct genus of cephalopods), mineralized as Calcite (CaCO3) in Fossiliferous Limestone

AI-generated identification · may be incorrect

Hardness: 3-4 (Mohs scale), Color: Black matrix with white/grey fossil inclusions, Luster: Vitreous to dull, Crystal Structure: Trigonal (as calcite replacement), Cleavage: Perfect rhombohedral

Identified March 20, 2026More sedimentary →

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Physical properties

Hardness: 3-4 (Mohs scale), Color: Black matrix with white/grey fossil inclusions, Luster: Vitreous to dull, Crystal Structure: Trigonal (as calcite replacement), Cleavage: Perfect rhombohedral

Formation & geological history

Formed during the Ordovician and Devonian periods (approx. 400-500 million years ago). These were marine cephalopods whose shells settled on the ocean floor and were buried under sediments, eventually fossilizing through mineral replacement in lime-rich mud.

Uses & applications

Commonly used for ornamental purposes, jewelry (pendants/cabochons), interior design (tiles and coasters), and as educational fossils for collectors.

Geological facts

Orthoceras were ancestors to the modern squid and lived inside long, straight, conical shells. They were among the most advanced invertebrates of their time, using gas-filled chambers in their shells to maintain buoyancy.

Field identification & locations

Identify by looking for white, bullet-shaped or ladder-like structures embedded in a dark grey-to-black polished rock. They are most abundantly found and commercially mined in the Anti-Atlas Mountains of Morocco.