Petrified Wood, polished on one side. Measures 6 1/2"x4". From Holbrook Arizona area. From the late Triassic epoch of the Mesozoic era, about 225 million years ago. Fallen trees near rivers were buried by sediments containing volcanic ash. Ground water dissolved silica from the ash and carried it into logs where it formed quartz crystals that gradually replaced the organic matter. Traces of iron oxide and other substances combined with silica to give varied colors. Most are an extinct conifer tree. Araucarioxylox Arizonicum.