What creatures are hurt by obsidian?

[Rock of the Fraternity]

Agreed.

I seem to recall that special materials even had to be consecrated or otherwise special in order to have any effect against werebeasts, according to some of the mythology. Imagine the frustration of having the right metal or other material readily to hand, but lacking a priestly blessing or the aura of three generations of possession to make your weapon effective against the monster currently ripping apart your front door...