sedimentary

Orthoceratid Fossil in Limestone

Orthoceras (extinct genus of Nautiloid Cephalopod)

Hardness: 3-4 (matrix), 5-6 (fossil replacement); Color: Grey, tan matrix with black or dark grey fossil inclusions; Luster: Dull to earthy matrix; Crystal structure: Microcrystalline calcite (matrix); Specific gravity: ~2.7

Hardness: 3-4 (matrix), 5-6 (fossil replacement)
Color: Grey, tan matrix with black or dark grey fossil inclusions
Luster: Dull to earthy matrix

Identified May 2, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed during the Ordovician period (approx. 485 to 443 million years ago) in shallow marine environments where the conical shells of cephalopods settled into soft lime mud (micrite) and were later mineralized.

Uses & applications

Primarily used as decorative stones, educational specimens for paleontologists, and as unique items for mineral and fossil collectors.

Geological facts

Orthoceras means 'straight horn'. These creatures are ancestors to the modern squid and lived inside their shells, using gas chambers to regulate buoyancy in ancient oceans.

Field identification & locations

Identified by the distinct conical, arrow-like shape of the shell section embedded in a sedimentary matrix. Commonly found in fossil-rich limestone beds across North Africa (Morocco) and parts of Europe and North America.

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