|Petrified wood (the now more popular name fossilized wood) is a replacement product. The process requires an organic material (wood, bush, fern, etc.) to die and quickly arrive in a nearly oxygen free environment. Usually the wood is buried under sediment but can also be submerged in low oxygen content water.
In either case the removal of oxygen impedes the organic materials destruction by bacteria. The isolated organic material can then be replace by dissolved silica over a very long period of time. The replacement is so exact that the original cell structure of the organic is at least partially maintained. Hence in the case of wood, there are tree rings, knots, bark, and even wood cells. Fossilized palm shows the elongated cell structure in palm trunks.
Some wood by-products are also found fossilized, like acorns, pine cones and seeds. The silica replacement color is dependent upon the minerals in the surrounding soil or those dissolved in the silica solution, so fossil wood may have the same general colors that jaspers and agates have. Cracks or other openings in the wood may contain agate fortifications.
Virtually any organic material can become fossilized. Two stones touched upon in the jasper section are Gary Green Jasper (aka larsonite) which is fossilized swamp bog, and Mary Ellen Jasper, fossilized stromatolite. Fossilized giant ferns are also found and cut into cabochons
Bones can also be fossilized by silica replacement of the Ca minerals. Hence fossilized dinosaur bones are fairly abundant in Utah and Colorado. (Getting on my soapbox for a moment, it is unfortunate that we may no longer collect agatized dinosaur one because of a Federal law that was designed to protect fossil animal remains in place like Bad Lands of South Dakota. I understand the need to protect full skeleton remains for scientific research, but agatized bone fragments in places like Utah are not likely to be systematically studied as there is little academic information in these.)
The fossilized dinosaur bones (sometimes called dino-bone) show the interior cell structure of the bone marrow, and there may also be agatized fortification agate in cracks or fissures The right reds, oranges and yellow are the most popular materials. Coprolite is the official name for another dinosaur related fossil, also known as "dino-poo". Coprolite is fossilize dinosaur dung, and interestingly enough can still be legally collected.
Shells and or the skeletons sea-water animals are also known. Petrified coral has been found in numerous locales around the world. Some of he fossil corals are cut across their diameters and reveal ring patterns similar to fossil wood. A beautiful red variety as been coming from Utah, and some striking materials have been imported recently from Indonesia.
A slight variation of fossilization sometimes results in opal instead of cryptocystalline silica becoming the fossilization agent. Hence there is opalized wood, and opalized shell material found in numerous locations. Virgin Valley Nevada is well known for its opalized wood, but the majority (not all) of the fire-containing wood will lose its firey color on natural drying.
Not covered in this section, but of interest are the replacement products of pyrite in both shell structures and fossil sea animal remains. Pyrite replaced shell material has been use in the jewelry trade.