Rock Identifier
Orthoceras fossil in limestone (Orthoceras regulare (extinct nautiloid cephalopod)) — fossil
fossil

Orthoceras fossil in limestone

Orthoceras regulare (extinct nautiloid cephalopod)

AI-generated identification · may be incorrect

Hardness: 3-4 (matrix); Color: Black or dark grey matrix with white/grey fossil inclusions; Luster: Sub-vitreous to dull; Crystal structure: Amorphous to microcrystalline calcite; Specific gravity: ~2.7

Hardness
3-4 (matrix)
Color
Black or dark grey matrix with white/grey fossil inclusions
Luster
Sub-vitreous to dull
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Physical properties

Hardness: 3-4 (matrix); Color: Black or dark grey matrix with white/grey fossil inclusions; Luster: Sub-vitreous to dull; Crystal structure: Amorphous to microcrystalline calcite; Specific gravity: ~2.7

Formation & geological history

Formed during the Ordovician period (approx. 488 to 443 million years ago) in marine environments where the conical shells of dead cephalopods settled into soft mud that mineralized into limestone.

Uses & applications

Commonly used for ornamental purposes, jewelry, home decor (coasters, bookends), and as educational specimens for fossil collectors.

Geological facts

The name 'Orthoceras' means 'straight horn,' referring to its characteristic long, straight shell. It is an ancestor of the modern-day squid and nautilus.

Field identification & locations

Identify by looking for white, bullet-shaped or cone-shaped patterns with visible internal chambers (septa) embedded in a dark stone. Most commercial specimens are sourced from the Atlas Mountains in Morocco.

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