Wondrous Item, rare
If the stone is touching the ground, you can use an action to speak its command word and summon an earth elemental, as if you had cast the conjure elemental spell. The stone can't be used this way again until the next dawn. The stone weighs 5 pounds.
Notes: Summoning
Does this item require concentration? ‘As if you had cast’ ... is what the text says. Usually items just say ‘you can cast’.
I'd say it does. "As if you had cast" implies that you follow the conditions of the spell. It doesn't count as casting the spell though so it can't be counterspell'd and doesn't require the V/S/M components or 1 minute to cast. The stone does all the work for you.
But if the stone does all the work, wouldn't the stone be the one "concentrating" on it?
Most items (all that I can think of) don't handle concentration. I think it would be really powerful if it did since there would be no downsides to using it. No components and no spell slot used up is already excellent. With no concentration required the user wouldn't even have to worry about losing control of the elemental (which they do if their concentration is broken). I could see a homebrew version with sentience concentrating on the spell but I'd give it control of the summon so the user has to keep the stone on friendly terms with them. That's my ruling on it. You can run it however you like in your game obviously :)
Does anyone know if there are any established rules to create a genie prison and trap the genie (say Dao) in it?
Planar Binding can be used as a spell I think, but still it's only my assumption.
there's the magic circle spell. You could also homebrew something like the efreeti bottle but for a Dao.