A black sapphire (but not a normal sapphire) could be used to enhance the effects of a wand, that contained a spell of the subschool of darkness. Only one could be built in per wand and once built in the wand's darkness spell magic either got harder for the target to resist or the strength behind the spell increased. Effectively, adding a black sapphire was a way for the crafter to substitute his lack of skill and need to burn his essence to create a strong darkness-wand with money (5000 gold as mentioned above).