Bone

Bone (BO) is a reagent that is in use on Pagan. Bone -speciafically human bone- is easy to find, although a little morbid.

Pagan
For the magic school of Necromancy, bone was a very easy to gather reagent, as their line of work automatically got them into contact with human remains. They invoke the powers of summoning and communication in a spell cast by the necromancer. Thus it mostly is used in spells that call undead or speak with the dead.

Ultima Online
When the Stygian Abyss expansion opened, it was found that the gargoyle Mysticism school of magic makes use of bone as a reagent.