The statistics for stone are not collected anywhere, but you can put useful things together …
- The stone is mentioned explicitly a few times in the DMG
section on objects:
Most useful – the AC for stone is given as 17.
Use common sense to determine the success of a character in damaging an object. Can a fighter cut a section of a stone wall with a sword? No, the sword may break before the wall.
You might decide that certain types of damage are more effective against a particular object or substance than others. (…) blunt damage works well for breaking objects, but not for cutting through rope or leather. (…) A pick can flake off Pierre but cannot effectively cut down a tree. As always, use your best judgment.
I know "use your best judgment" isn't the takeaway you are looking for here
but i would apply these instructions to give resistance to the stone against drilling and cutting damage but not clubbing. As you quoted “ objects are immune to poison and psychic damage '', but I would also give resistance to stones like fire damage and necrosis. How you apply your judgment is up to you, so I'm not going to list all types of damage here.
- The guidelines on HP are not self-explanatory but could probably be adapted to a stone wall:
A stone wall is normally considered resilient and (at least) large, which would add at least 27 hp (5d10) and possibly much more – although in the case of a wall, you can affect 27 ch at each specific section of the wall (see section on huge and gargantuan objects).
- You can use the damage thresholds of the published modules
DMG does not provide explicit guidelines on appropriate damage thresholds. However, other published supplements provide damage thresholds that can be referred to as a point of comparison:
- The larger boat hulls in Ghosts of Saltmarsh (and now available through the ground rules) have damage thresholds between 10 and 20.
- A submarine in Waterdeep: Dragon Heist has a damage threshold of 15, while it gives steel doors a threshold of 10
- The infernal machines and chains of Baldur & # 39; s Gate: Descent in Avernus all have a threshold of 10.
- In Tomb of Annihilation, there is a crystal window with a damage threshold of 15 and more chains with a threshold of 10.
Considering all the above, a damage threshold of 15 to 20 seems appropriate for the stone.
In addition to the concerns of balance and likelihood of the game, when setting the damage threshold, you can also consider exactly how hard you want the stone to be for your players to destroy, depending on the current level . At low levels, a threshold of 10 may be enough to make your players understand the difficulty of destroying a wall, without completely eliminating the possibility of doing so.