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.
It's hard to make them spawn in artificial biomes. Like all other Treasure Chests they can only spawn on solid natural blocks in darkness, but will stay where they are when you place torches next to them or when the sun rises after they have already appeared.
You cannot put anything inside these special chests, and if you take "everything", then you will not receive any chests or the like, but only the contents of the chest, while the chest itself will be gone too.
Unlooted Treasure Chests will vanish if all player-characters wander off too far. Half-looted chests will prevail a little longer so you can teleport home and return within a short while, but will disappear if player-characters do not return to it for a longer time or log out.
Stone Treasure Chests can - randomly - contain (as of R26 in December 2015):
- Notes and/or Data Chips
- Perfumes (Eau De Pigsy, Fantasia De Leafi, Arome Chizzard, L’Rockster)
- Pet Accessoires like Hats or Bows
- Moss Torches
- Wood Mining Cells
- Basic Extractors
- Wood Swords
- (blocks of) Blue Carpet
- Explosive Bombs
- (blocks of) Wood Walls
- Melted Wax
- Wood Swords
- and occasionally crafting-recipes