It's a good idea to save every Blacksmith's Whetstone and Arcanist's Ether Path of Exile 2 players find in the Stash until the endgame.

Blacksmith's Whetstone is a currency item in Path of Exile 2, which can be used on martial weapons to improve their quality. This will be essential throughout the game, particularly in the endgame ...

KCD2’s Legacy of the Forge DLC may start with rebuilding Martin’s smithy, but it could easily turn into one of the game’s deepest endgame systems.