Caustic weapon imbues the targeted weapon with a shimmering stone-like field which grants the weapon a magic enhancement that deals an additional +1d6 acid damage on each successful strike. (If the weapon is not magical, it also counts as a magic weapon for purposes of bypassing DR for the duration of the spell.) This spell fails if cast upon a weapon that already deals acid damage. It cannot be cast on most natural weapons or a collection of ammunition, though it can be cast on a ranged weapon, imbuing all missiles fired from the weapon the additional damage for the spell’s duration. It can be cast on the natural weapons of a creature of the earth subtype.