Stonewashed blades from the catalog
A stonewash finish is a blade surface roughened by stones: Kershaw says it tumbles its stonewashed blades with ceramic stones, and Boker describes the blade being "washed" in a drum holding different types of stones. The result is a scuffed look that Kershaw says helps hide scratches and fingerprints and that Boker says has less of a sheen.
Kershaw says its stonewashed blades are tumbled with ceramic "stones", which gives the blade surface a roughened or scuffed look. Boker describes the same idea as controlled scratching: the blade is "washed" inside a drum of different stones, and the scraping and falling stones make the effect.
Kershaw's bead-blasted finish, sprayed with fine media rather than tumbled, is a different process, covered under bead blast finish.
Kershaw says the stonewashed finish helps hide scratches and fingerprints. The reason is visual: a surface already covered in scuffs gives a new scratch or a fingerprint less contrast to stand out against. Boker makes the other half of the point about scratches and signs of wear: on a polished blade, "such flaws would be rather conspicuous".
Boker also says stonewashed blades have less of a sheen.
A plain stonewash finish adds no layer to the blade. The stones mark the steel that is already there, so the texture is the steel itself rather than something sitting on top of it. Dark variants are a separate case: Kershaw says its products may carry a DLC BlackWash or a black-oxide BlackWash finish, and anything with a coating belongs with blade coatings.
The stonewash texture says nothing about the alloy under it: the spec tables print stonewash finish beside D2 tool steel on one listing and S30V stainless on another, and the Steel Lab covers the steel. Whether to pick stonewash over satin is answered in stonewash or satin finish.