"GeoPicture of the week: Tree fossil with opal growth rings -- 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. …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."
Ethiopian Opal rough and cut by Eva
I've always thought geodes were really cool. Our daughter brought this one back from South Carolina. It makes for a beautiful paperweight.