Resembling one of the coral ringed islands with a cone on top that form as dying volcanoes sink and corals build up around them and whose mode of formation was deduced by Lyell and Darwin, this piece of agate is actually a slice of petrified palm trunk. The woody matter has been replaced by silica, and the bands show how it grew into the type of stone known as fortification agate (for obvious reasons). It was preserved in one of the best fossiliferous formations in the USA, the Eocene (56-34 million years ago) Green River formation.
Deposited in between the rising Rockies as swampy lake basins, they also contain excellently preserved plants and fish (see past fossils from these rocks at http://on.fb.me/1DlFXfw and http://on.fb.me/1FTtQvU). Layers of interleaved volcanic ash in the sediments have allowed us to date them as being deposited between 53.5 and 48.5 million years ago. The slice is 20cm across.
Image credit: LGF Foundation.