Originally posted by morrokain10587:

Swamp key is non renewable since withered bones are non renewable - though very plentiful.

Swamp key drops from The Elder, who is summoned via Ancient Seeds. You can make an arbitrarily large number of them, but possessing more than one per character grants no additional benefit.

Bonemass is summoned via withered bones (which are finite)... but the Wishbone (the non-trophy drop from that boss) is another object which grants no additional benefit from possessing more than one per character.