Though it does not have the iridescence associated with the commonly-known forms of opal, this material from Bulgaria is referred to as "wood-opal" for its fine, dense texture and mineralization by silica, replacing the original wood. You can see the very bark-like exterior still intact, and the center has been completely mineralized. This specimen came out of an Eastern European museum collection, as you can see from the label. Rare in this quality! 12.1 x 10.8 x 5.9cm