The Arkenstone of Thrain
Permanent light-source (radius 3). It increases resistances: +20% light
and +20% dark. It provides protection from the effects of experience
drain. It gives its wielder see invisible. It cannot be damaged by any
means. When worn and activated, it detects everything of interest on the
panel, recharging every 30+d30 turns.
Level 50, Rarity 50, 0.5 lbs, 25000 Gold.

(EyAngband 0.5.2)