Chaledonized gastropods (specimen at far left is 3.0 cm tall) - these snail shells have been replaced by translucent, fibrous, microcrystalline quartz (SiO2). This variety of quartz is called chalcedony. Chalcedonization is a variety of silicification. Chalcedonized fossils are uncommon.

Age: Cretaceous (apparently)

Locality: unrecorded, variously attributed to either Dakhla, “Western Sahara” (= southern Morocco) or the Gobi Desert of eastern Asia.

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The most common replacement mineral is quartz (silica) (SiO2) - fossils that have been replaced by quartz are said to be silicified (silicification).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.