Glittering Beljuril Gemstone

The Glittering Beljuril Gemstone is the item described by one of the four dwarves guarding the stairs to the second underground floor of Durlag's Tower. It can be found in the chamber with a lot of treasure near the area with chest containing The Practical Defense.

In-game description
"This stone seems to be complety out of place down here, almost as if it wasn't a coincidence after all. Perhaps it has some meaning, maybe even a purpose here. You get an uneasy feeling that this gem shouldn't be sold, perhaps it would be best not to be greedy and keep it around a while. Normally a deep, pleasant seawater green, beljurils periodically blaze with a glittering, winking, flashing light. Although this particular stone has a slightly different hue, perhaps a new strain, rarely if ever seen before. It looks incredible valuable."