Illithium Ore

Illithium Ore is an item needed in the side quest: Gain the Services of Sir Sarles for the Temple. You need to bring this to Sir Sarles in order to convince him. Afterwards, you discover that he tried to give you an impossible task to get rid of you. This item can be gained by killing Neb in the derelict house in the Athkatla Bridge disctrict. This item appears in Baldur's Gate II: Shadows of Amn and Baldur's Gate II: Enhanced Edition.

This item might also be used to upgrade Mace of Disruption by Cromwell or someone else, but this is not confirmed yet.

In-game description
"This is 200 stones of pure illithium ore. Illithium is renowned for both its luster and its unique property of never becoming soiled."