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 by chalcedony-filled fractures that crosscut permineralized tissues in some specimens. Spaces between adjacent tracheids commonly remain unmineralized, causing the silicified wood to remain permeable to water, and to readily cleave radially and tangentially. To a lesser degree, the fossilized wood is subject to transverse fracturing. This combination of structural characteristics causes Florissant fossil stumps to be susceptible to damage from freeze-thaw weathering.