Wood opals are moderately valuable gems that can be found just about anywhere.
 In Real Life
Wood opals are a variety of petrified wood whose structure happened to transition into opal instead of the more usual chalcedony. The amount of actual opal on the fossil can be as little as a thin shiny coating, but the in-game versions probably represent the much more valuable and uncommon solid opal specimens.
[SOLID_DENSITY:2115] Common to opals. Range is 1980 - 2250