... a photographic technique of using a reflector on one side and a damper on the other to control the fall of light onto the rock might work.

Your reflector need be no more than a sheet of silver or gold card (cost about a quid from an art materials shop) on one side and a sheet of matt black card (cost about a quid from an art materials shop). Simply hold them in place, or peg them or get someone else to hold them, until you have brought up the relief on the stones. This way you don't even have to touch the stones.

Using reflectors would work even better if you watered the stones first.

Good luck!