#opalised fossil

Unexpectedly morbid quartet

Opalized fossils are the result of organic materials such as bone or shell being replaced by opal, which is an hydrated amorphous silica that diffracts light in a characteristic manner. Many opalized fossils, and most opals generally, seem to come from Australia. I’m not a geologist or paleontologist, so I will let an expert describe further: “The process of mineralisation of bone and shell for preservation in the fossil record is dependant on the chemical and physical conditions during diagenesis, particularly the composition of the mineralizing fluid. Fundamentally these processes can be considered as mineral replacement reactions, where the biomineral is replaced by another mineral such as quartz, opal or calcite.

[…] The processes leading to the formation of precious opal and the opalisation of fossils in the Australian opal deposits are poorly understood. […] Precious and potch opal are also found replacing, shells, wood and less frequently, the bones and teeth of marine reptiles (Barnes et al. 1992). The skeletons of Cretaceous plesiosaurs and ichthyosaurs that have been partially or completely opalized have been found at Andamooka, Coober Pedy and White Cliffs (Kear 2003).” 

- Opalisation of fossil bone and wood: clues to the formation of precious opal. B Pewkliang, A Pring, J Brugger - Regolith, CRC LEME, Australia, 2004 Basically, these fossils can be both gorgeous and paleontologically valuable! Photos: Opalized petrified wood - PlanetOpal

Opalized teeth and bones - Gemological Institute Of America

Opalized belemnite (cephalopod) - Wikimedia

Opalized bivalves -  Gemological Institute Of America

Opalized fish teeth -  Gemological Institute Of America

Opalized plesiosaur vertebra - James St. John

Opalized bivalves - Wikimedia

Opalized belemnite (cephalopod) - Wikimedia

Opalized bivalve - James St. John