Obtained rarely when mining boulders or stones, will have the same purity as the boulder it was obtained from. With it, you can sharpen any Sword or Halberd. In this way, weapons can have Purity which increases the amount of damage.
A whetstone can be used on all swords and halberds, such as Spanish Sabre, Adventurer's Rapier, Claymore, Nobleman's Rapier, Nobleman's Rapier, or Pirate Captain's Sword, Halberd, Executioner's Axe, and Golden Halberd.
Beware, this counts as Any Stone in recipes and can also be paved.