Grindstone

Grindstones are used to gain temporary enhancements for weapons which allow them to deal 20% more damage. They can typically be found where there are blacksmiths or armorers, but the presence of these craftsmen is not strictly necessary. With the Hearts of Stone expansion, enchantments were introduced, among which is Preservation which makes the enhancement provided by grindstones permanent.

White Orchard
There are 4 grindstones:
 * 1 in the Nilfgaardian Garrison
 * 2 at Willis's yard in White Orchard village
 * 1 at the Sawmill

Pontar (Velen and Novigrad)
There are 14 grindstones:

Skellige
There are 10 grindstones:

Kaer Morhen
There are 3 grindstones:
 * 2 in the courtyard of Kaer Morhen
 * 1 inside the keep

Grindstones increase damage inflicted by 20% and may be used on either silver or steel blades.