sedimentary

Orthoceras Fossil Limestone

Orthoceras (extinct genus of cephalopod); Limestone matrix (CaCO3)

Hardness: 3-4 (Mohs); Color: Black or dark grey matrix with white to light-grey fossils; Luster: Polished/Vitreous; Structure: Massive limestone containing fossilized internal molds; Specific Gravity: Approx 2.7

Hardness: 3-4 (Mohs)
Color: Black or dark grey matrix with white to light-grey fossils
Luster: Polished/Vitreous

Identified March 11, 2026More sedimentary →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed in marine environments during the Ordovician period (approx. 400-500 million years ago). The soft-bodied cephalopods died and their shells were buried in seafloor sediment, eventually mineralizing into calcite or aragonite within limestone.

Uses & applications

Primarily used for decorative items, jewelry (cabochons), paperweights, and educational fossil specimens. In some regions, large slabs are used for architectural tiling.

Geological facts

Orthoceras means 'straight horn,' referring to the characteristic long, conical shell. They are ancestors of the modern squid and lived in the seas long before the first dinosaurs appeared.

Field identification & locations

Identified by the distinct white, cone-shaped or bullet-shaped inclusions against a dark background, often showing internal chambers (septa). Most commercial specimens originate from the Atlas Mountains in Morocco.

Other sedimentary specimens

See all

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