Opalized Belemnite Fossil

An opalized belemnite fossil is a rare and fascinating gem that combines the beauty of opal with the fossilized remains of an ancient marine creature. Belemnites were squid-like cephalopods that lived in the oceans during the Jurassic and Cretaceous periods. Over millions of years, their shells were preserved and replaced by opal, creating stunning specimens that showcase the intricate patterns of the fossilized organisms.

Belemnites: Marine Creatures of the Past

Belemnites were an abundant group of marine cephalopods that lived during the Jurassic and Cretaceous periods, ranging from approximately 200 to 145 million years ago. They were closely related to modern squid and cuttlefish, characterized by an elongated, cigar-shaped shell and a prominent phragmocone, a series of gas-filled chambers that facilitated buoyancy and movement.

Opalized Belemnite Fossil. Rare opalized belemnites in matrix from Australia-based True Blue Opals. (Photo: Sally Patel)

Belemnites declined through the Late Cretaceous, and their range became more restricted to the polar regions; the southern populations became extinct in the early Maastrichtian, and the last belemnites—of the family Belemnitellidae—inhabited what is now northern Europe.

They finally became extinct in the Cretaceous–Paleogene extinction event, around 66 mya, where, like in ammonites, it is thought the protoconch of embryos could not survive the ensuing acidification of the oceans

Opalized Belemnite. Provenance: Coober Pedy, Australia. age: Upper Cretaceous, 95 million years. Photo: © Alias Collections

How Does A Belemnite Become Opalized?

Opalization, the process that transforms belemnites into opal fossils.

When a belemnite died it would sink to the bottom of the sea and the sediment on the floor would be very muddy from the clay based soils. The decaying body would have been covered in this clay sediment very rapidly and stayed there for millions of years in desert type conditions.

After the sea eventually dried up the acidity in the sandstone soil greatly increased and the silica gel from the sandstone escaped and filled in crevices in the clay and in some areas these belemnites were encased in clay.

Over millions of years the acidic levels dropped and the silica gel hardened into opal in impressions of the decayed squid. This process was not common and most belemnites do not have opal colours. It is extremely rare to find multi colour opal or black crystal opal like the virgin rainbow.

It is understanding the rarity of this opal that makes one appreciate how rare and unique it is for mother nature to have produced such a spectacular belemnite opal.