The tune banite ore scroll is used during the Ritual of the Mahjarrat quest. It is found by searching a bookcase in the underground area in Kethsi unlocked with the Kethsian key.
Reading the scroll unlocks the Tune Banite Ore spell on the Lunar spellbook. The scroll is destroyed on learning the spell.
Learning the spell is a requirement for the completionist cape.
If you forgot to grab the scroll before leaving or finishing the quest, just make your way around the agility course, and when you get to the end slide down the platform and then down the stairs and through the door and search the bookshelf in the south-east corner.