This is a part of an opalized tree, and the rings you see are actually tree rings; or at least they were.
This is an very rare sample: opal can be fairly common in petrified wood, but this is a fire opal, which makes it so much more valuable. Fire opals are transparent to translucent opals with warm body colors of yellow, orange, orange-yellow or red – basically, it often has pretty colors.
So how did this magnificent opalized tree came to be? Initially, water filled the cracks and empty spaces of the source fossil, and the silica content in the water hardened and turned into opal or chalcedony.
The above tree fossil is from Queensland, Australia; it measures 25x22x5mm and weighs 23.5 ct.