Photo: David Medcalf, CC BY-SA 2.0, via Wikimedia Commons
It’s not clear whether the stones were deliberately placed. Either way, two surface tags come to mind:
surface=rock: the wiki notes it “is applicable both in some cases of man-made surfaces of trails and in cases where trail goes across exposed bare rock.”
surface=stepping_stones: defined as “individually placed stones set at regular intervals for stepping from one to the next, to avoid walking on the ground below. Commonly used for river or creek crossings, muddy crossings, or areas to preserve the grass from being trampled.”
surface=rock only because I try to avoid using infrequently used tag values. Much more important is to tag sac_scale=mountain_hiking or maybe smoothness=horrible if the way is likely to be used by wheeled vehicles (as long as it’s firm, I don’t care much what the surface under my feet is called)
surface=rock + highway=steps was my first thought too.
highway=path + surface=stepping_stones also feels reasonable, although stepping stones on an incline basically sounds like the definition of (rudimentary) steps!
