Stones found amongst streams, rivers and lakes fit the requirement as a Primitive Mimic for a sharpening stone. Truly all that must be found is a smooth stone with a flat spot. Sharpening in the same way as a handheld stone makes quick work of sharpening a knife's edge, I use small circles along my knife's edge, alternating sides until my edge is ferocious. That's it, done deal.
...If ever your found in the wilds without your sharpening stone, never forget whence sharpening stones originated.