Fossil wood from the Triassic of Arizona, USA. (cut & polished slice, CMNH P-26408, Cleveland Museum of Natural History, Cleveland, Ohio, USA)

This beautiful fossil is from the Upper Triassic Chinle Formation, a multicolored, nonmarine sedimentary unit consisting of coarse-grained siliciclastics (fluvial channel facies) and fine-grained siliciclastics (floodplain facies). Quartz-permineralized fossil wood is famously common in the Chinle Formation (for example, Petrified Forest National Park in Arizona).

"Petrified wood" refers to fossil wood that, while buried in sediments, had its microscopic porosity filled with minerals as groundwater percolated through. This fossilization process is technically known as permineralization ("petrified" is a horrible term - never use it in a scientific context). Permineralization can also occur with fossil bones. The most common permineralization mineral is quartz (~pure silica - SiO2), which is the most common mineral in Earth's crust.

Stratigraphy: Chinle Formation, Upper Triassic