sedimentary
Orthoceras Fossil in Limestone
Orthoceras (extinct genus of cephalopods), mineralized as Calcite (CaCO3) in Fossiliferous Limestone
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 More sedimentary →
Identify your own rocks.
Get a report just like this from any photo, free.
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.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Sandstone
Arenite (primarily SiO2)
sedimentary
Shale or Slaty Mudstone
Argillaceous sedimentary rock
sedimentary
Chert Breccia
Brecciated Chert (Microcrystalline Silica)
sedimentary
Sandstone (Quartz-rich)
Arenite (SiO2)
sedimentary
Sandstone (with man-made markings)
Arenite (composed primarily of Quartz, SiO2)
sedimentary