sedimentary
Orthoceras Fossil in Limestone
Orthoceras regulare
Hardness: 3-4 (Mohs scale), Color: Black matrix with white/grey fossil, Luster: Polished/Dull, Crystal Structure: Microcrystalline calcite matrix, Cleavage: None (concretionary)
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, Luster: Polished/Dull, Crystal Structure: Microcrystalline calcite matrix, Cleavage: None (concretionary)
Formation & geological history
Formed during the Ordovician period (approx. 488-443 million years ago) when the shells of cephalopods settled on the sea floor and were covered by sediment, eventually turning into limestone.
Uses & applications
Primarily used for decorative purposes, jewelry (pendants/cabochons), paperweights, and educational fossil collecting.
Geological facts
Orthoceras were ancient mollusks related to the modern squid and nautilus; they had straight, cone-shaped shells and could grow from a few centimeters to over 10 feet long.
Field identification & locations
Identifiable by the distinct white conical chambers against a dark limestone background; most common specimens available today are sourced from the Atlas Mountains in 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