English: "Opalized plesiosaur vertebra from Australia. (Field Museum of Natural History, Chicago, Illinois, USA)

Replacement is a fossil preservation style involving the crystal structure and the mineral of an organism's hard parts being changed.

The most common replacement mineral is quartz (silica) (SiO2) - fossils that have been replaced by quartz are said to be silicified (silicification). Many silicified fossils have rounded to pustulose structures covering their surfaces. These are called beekite rings, but they're composed of ordinary quartz.

Other common replacment materials include the mineral pyrite (FeS2 - iron sulfide) and calcium phosphate. These replacement styles are called pyritization and phosphatization.

Numerous other minerals have been found replacing minerals - many of them are quite rare. Reported fossil replacement minerals include: anglesite, apatite, barite, calamine, calcite, cassiterite, celestite, cerargyrite, cerussite, chalcocite, cinnabar, copper, dolomite, fluorite, galena, garnet, glauconite, gumbelite, gypsum, hematite, kaolinite, limonite, magnesite, malachite, marcasite, margarite, opal, pyrite, romanechite/psilomelane, siderite, silica/quartz, silver, smithsonite, specular hematite, sphalerite, sulfur, uranium minerals, and vivianite.

(List mostly from info. in Hartzell, 1906 and Klein & Hurlbut, 1985)

Opal (SiO2·nH2O) is a rare replacement mineral. The fossil bone shown above has been replaced by colorful precious opal. The museum display had no specific locality information, but this specimen may be from the famous Coober Pedy Opal Field in South Australia. There, the remains of several groups of organisms are known with their hard parts replaced by precious opal, including bivalves, gastropods, belemnites, crinoids, ichthyosaurs, and plesiosaurs.

This bone is a vertebra from a plesiosaur, which was a large, predatory, marine reptile that existed during the Mesozoic. Plesiosaurs had large bodies, two pairs of flippers for swimming, and a long neck. They resemble the stereotypical image of the "Lake Ness Monster" ("Loch Ness Monster"). Plesiosaurs went extinct during the Cretaceous-Tertiary mass extinction 65 million years ago.

Locality: unrecorded/undisclosed site in Australia" -James St John