This is a 11.5" wide petrified wood round from Hampton Butte, Oregon. It's polished to a mirror like finish one one side and is a pretty unusual piece.
This round was part of a old petrified wood collection that I recently bought containing petrified wood from many unusual locations. Be sure to keep an eye out for more great specimens.
Petrified wood is the name given to wood that has been turned into stone (fossilized) through the process of permineralization. All of the organic matter becomes replaced by minerals, while much of the original structure such as tree rings in retained. For this to happen the wood needs to be buried in an environment both low in oxygen (preventing decomposition) and with flowing, mineral-laden water.