Stone Treasure Chests can be found on the Fossil layer underground and shallow caves that will lead into this layer, where they will spawn randomly at any time of (ingame-)days as soon as player-characters come into range of vies (no direct line of sight needed though) on Limestone or Bedrock, more rarely on Nodes of Coal or other natural blocks. They don't spawn on ordinary grey Stone though, that's why you cannot find them in the Mountains. Stone Treasure Chests can - randomly - contain (as of R26 in December 2015):