Ring of Distance (3.5e Equipment)
From D&D Wiki
This magic ring in will detach into two, separate rings. The wearer of one ring will know the exact distance to the other ring, but not any other information about it. This distance is only known when the paired ring exists on the same plane of existance (Except for overlapping planes, e.g. Ethereal, in which case the distance is reported normally). If one of the two rings is destroyed, both rings will immediately turn to dust.
Market price: 30,000 gp; Weight: 1 lb. (per ring)