I didn't find this gem, and don't recall seeing it mentioned in the forum, so it is most likely not for anything in particular (just loot).
Some items in inaccessible locations can be grabbed with Telekinesis, or by tossing a teleporter stone beside it, teleporting there, picking the item and teleporter stone up and teleporting back (useful for a spell book in the sewers, for example). If there are monsters where you can not reach (such as on one of the mountains in the Dark Forest) you can use Flash Attack to get there, but leave a monster alive so you can get back (unless you have a teleporter stone with you).
You can not walk in many normally inaccessible regions (like across the river in the south east corner of the dark forest), but can move around by repeatedly tossing one stone and teleporting to it.
