fossil

Orthoceras Fossil in Limestone

Orthoceras sp. in Bituminous / Organic Limestone (primarily Calcite, CaCO3)

AI-generated identification · may be incorrect

Hardness: 3 on Mohs scale; Color: Black to dark grey matrix with white to light grey fossil chambers; Luster: Vitreous to sub-vitreous (when polished); Cleavage: Perfect rhombohedral in calcite crystals; Specific Gravity: approximately 2.6 - 2.8

Hardness: 3 on Mohs scale
Luster: Vitreous to sub-vitreous (when polished)

Identified October 3, 2026More fossil →

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Physical properties

Formation & geological history

Formed during the Paleozoic Era (principally Ordovician to Silurian periods, approximately 400-450 million years ago) in shallow, warm marine environments where straight-shelled nautiloid cephalopods died and were buried under fine carbonate mud and organic matter.

Uses & applications

Extensively quarried and polished into decorative objects like eggs, spheres, plates, bookends, and tabletops. Also popular for educational collections and metaphysical decorative displays.

Geological facts

Orthoceras ('straight horn') were ancient mollusks related to modern squids, cuttlefish, and nautiluses. Their shells consisted of multiple chambers (camerae) linked by a central internal tube called a siphuncle, used to adjust buoyancy.

Field identification & locations

Most commonly quarried in the Atlas Mountains near Erfoud, Morocco. Field identification relies on the distinctive straight or conical chambered shells preserved in a dark, organic-rich limestone matrix that reacts strongly to dilute hydrochloric acid.