Silicified stumps preserved within a late Eocene lahar deposit have diverse mineralogy, ranging from opal-CT to chalcedony. In specimens that contain both silica polymorphs, the minerals appear to have originated independently, rather than from diagenetic transformation of an opaline parent material. This petrifaction process is unlike the progressive transformation of opal-A→opal-CT→chalcedony that has long been accepted as a general model for wood silicification. At the Florissant fossil forest, petrifaction occurred in several stages, beginning with precipitation of amorphous silica on cell wall surfaces. Cell lumina later became filled with opal-CT and chalcedony. A final phase of silica deposition is evidenced...