Why does a wet stone look darker than a dry one?

Because water changes where the light goes. A dry surface is rough, and light hitting it scatters straight back out in all directions, much of it reaching your eye. A film of water smooths the surface and, crucially, has a refractive index between air and stone - so light passes into the water, hits the stone, and much of what reflects off is bounced back down into the surface again instead of escaping.

Light that goes back in gets more chances to be absorbed. Less comes out, so the stone looks darker and its colour looks deeper and more saturated, which is why wet pebbles on a beach are beautiful and disappointing when they dry.

It is the same reason a coat of varnish or oil makes wood and paintings look richer, and why a stain on cloth is more visible wet. The exception proves the mechanism: materials that are already smooth and non-absorbent, like glass or glazed tile, barely change when wet, because there was no rough surface scattering light in the first place.