Given one sending stone, how can I locate its mate?

Question

Suppose I have one of a pair of sending stones, and I wish to locate the other stone of the pair. Is there any spell or other magic that will allow me to find the stone's mate? Assume I have never held or seen the stone's mate.

A limited range solution is potentially acceptable, but a longer range or unlimited range solution is preferred (even more so if it works across planes), since I have no way of knowing where the other stone is. However, any solution must locate the specific stone, not just any sending stone, not least because unless I have a bag of holding, the nearest sending stone is always going to be the one in my pocket. If there are any additional limitations to the solution, be sure to note them (e.g. "the stone must not be in the possession of a creature", "the stone must not be inside a lead box", etc.). Lastly, casting Wish and wishing to know the stone's location should obviously work, so there's no need to mention this option. If you like, consider Wish the "baseline" solution: your solution should be less demanding than one that requires a 9th level spell slot, multiple days of rest, and risking losing access to Wish. In short: the best solution would have the longest possible range and the fewest possible limitations.

One additional note: since a lone sending stone wouldn't be of much interest unless it was in someone's possession, you may assume that the stone is held by a creature, and a solution that locates the creature in possession of the stone is also acceptable, since this would also locate the stone. However, assume I have never met this creature and know nothing about them, other than that they possess the stone.

Answer 1

Just activate the one you have, and see what happens.

If its mate isn't in a creature's possession, activating your stone tells you this, and doesn't consume its daily charge. But if someone (or something) does possess it, then you two have a pair of sending stones. You can negotiate its return to you, offer to sell yours, or whatever will entice one of you to the other's location.

The worst the other creature can do is abandon its stone (or possibly destroy it, which renders your stone nonmagical), or just refuse to answer you.

Sending works over unlimited distance and even across planes (there's a 5% chance that the message never arrives, but the stones recharge each dawn), so distance isn't a problem. The sending spell itself also states that the only requirement for the creature to understand, recognize the sender if possible, and reply is at least 1 point in Intelligence, which even beasts possess.

It's not likely to work, but it's free.

Answer 2

If the stone's owner is talking back to you, then work your way toward scrying

Assuming that someone possesses the stone on the same plane as you, who is responsive to you but is unwilling to share their location, and assuming you have time (up to several days) to go about this, then I think you have a good chance at eventually scrying on them.

First let's consider the minimum knowledge requirement, about the target, to get scrying up and running. The scrying spell description states:

The target must make a Wisdom saving throw, which is modified by how well you know the target...

Knowledge       Save Modifier

Secondhand (you have heard of the target) +5

So you don't have to have met him, just "have heard of" him, and there is no restriction on how you have "heard of the target", meaning you could have heard of him from the target himself, via a sending stone conversation. So, if you can get him to tell you enough about himself that it would pass as having heard of him, then you're in business.

Note furthermore that the scrying spell description does not require that you need to know the target's name. No reason you can't declare to your GM "I've heard of a guy who....[unique description]." Presumably, you could go to your GM ordinarily and say "I heard of a guy who ran away from a circus, then joined the cult of Dunatis, but now is retired" and that would be enough.

Or if your GM insists you at least know their name, well, maybe you can get the target to say at least his first name, and a few things about himself. Now you can say to the GM, "I know a guy named Grinnis who learned from his grandfather how to make barleywine." That should do it.

So how do you get the target to share this bit of information? I don't see why you can't ask the GM to let you make a Charisma (Persuasion) check when you cast sending, to persuade the target to be forthcoming about a few facts about himself. You have 25 words to say whatever you want, and words can be aimed to persuade. (Of course you could just try to persuade him to divulge his location, but I'm assuming that in this scenario the target is very stubborn about that -- the DC is super high for persuading them to just say where they are.) Generally, people love to talk about themselves, and in most cases I'd expect the GM would let you make some kind of attempt at getting the target to do so.

Anyway, once you pass the Peruasion check and you get enough info to "have heard of him", then you can start attempting scrying. Yes, the target will get a +5 on his save attempt, because you've never met him in person, but heck, you still get to try once every 24 hours. And I'd love your chances if you're a multiclass bard-sorcerer with Charisma 18, heightening your scrying.

The final step, obviously, is to hope that in scrying, you can recognize the target's location. Good luck!