This is a polished slab of beautifully detailed petrified wood from Arizona that has been made into a clock. The wood is Araucaria, a type of primitive conifer and is from the Upper Triassic Chinle Formation, the same formation as the famous petrified forest.
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.