This is a large, richly colored slab of petrified Araucaria (extinct Conifer) wood from Ambilobe, Madagascar. It is 19.2" x 14.25" and both sides have been polished showing off the amazing mineralization of this fossil. It is from the early Triassic period, approximately 220 million years old, close to the time the first dinosaurs appeared on the planet.
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.