Bronze Ioun Stone gives the wearer an extra lvl 7 spellslot. It can only be equipped by bards, mages and sorcerers. The stone weighs 2 lbs. The stone can also be used to upgrade the Circlet of Netheril.
This item appears in ToBBGII:EE(ToB), it can be gained after laying the ghost to rest in the Saradush old prison.