Opalized Belemnite Fossil

An amazing fossil where the opal and filled inside the empty cavity left by the fossil.

Belemnitida is an extinct order of cephalopods which existed during the Mesozoic era, from the Hettangian age of the Lower Jurassic to the Maastrichtian age of the Upper Cretaceous. The belemnite is the state fossil of Delaware.

Belemnite remains are found in what were littoral (nearshore) and mid-shelf zones. Squid and octopuses diversified and began to outcompete belemnites by the Late Jurassic to Early Cretaceous. 

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?

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.