This is a small limb section of Triassic aged (220 million years old) petrified wood from Ambilobe, Madagascar. It comes from an Araucaria tree, an extinct type of conifer. One face of this limb section has been polished to show of the inner wood detail.
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.