Araucarioxylon arizonicum: Arizona Agatized Wood
Arizona Petrified (Agatized) wood was once a forest of thriving conifers during the Triassic period when the continents were one big continent, Pangea and the earth had no polar icecaps.
The colorations in the stones come from the mineralization of the wood. Since the earth had no strong seasons (it was a balmy climate due to the continent's positioning at the equator) the rings you do see in the Agatized wood today have to do with the mineralization process.
These particular cuts have shades of reds, fushia, purples and yellows.