fossil

Fossil Belemnite Guard

Belemnitella spp. (rostrum)

Hardness: 3.5-4 on Mohs scale (calcite); Color: brownish, yellowish-tan, or grey; Luster: vitreous to dull; Crystal structure: hexagonal (calcite composition); Cleavage: perfect rhombohedral; Specific gravity: 2.71

Hardness: 3
Color: brownish, yellowish-tan, or grey
Luster: vitreous to dull

Identified August 4, 2026More fossil →

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Physical properties

Formation & geological history

Formed during the Mesozoic era (Jurassic to Cretaceous periods, roughly 200 to 66 million years ago) when the internal calcite skeleton (rostrum) of an extinct squid-like cephalopod became fossilized through mineral replacement in marine sediment.

Uses & applications

Used by paleontologists as index fossils for dating marine sedimentary strata and highly prized by fossil collectors.

Geological facts

Belemnites are closely related to modern squids and cuttlefish. Their bullet-shaped guards were commonly known as 'thunderstones' in folklore, as people historically believed they fell from the sky during lightning storms.

Field identification & locations

Easily identified by its distinctive bullet, dart, or cigar shape, solid calcite composition, and often showing a fibrous radial structure or a tiny apical canal when broken or sectioned. Commonly found in chalk deposits and marine limestone formations.