Some of the beds contain large amounts of a special clay mineral called bentonite. Bentonite is swelling clay, in that it absorbs water and expands, and when it dries out it contracts. Bentonite forms by the chemical alteration of volcanic ash. Thus, the sediment composing some of the beds was composed of volcanic ash. These beds, in the Chinlee, contains the majority of the petrified forests.
Petrified wood is actually rock formed by the gradual replacement of wood by minerals.
Mineral matter is deposited in and around cells of wood. Crystals or grains of silica are the mineral quartz, but the silica in petrified is so fine that you can't see the grains even with a microscope. Rock composed of cryptocrystalline quartz is called chert. So it ranges from white, to cream, to red, to black, to yellow. Red chert is jasper.