Create a sudsy solution of dish soap and hot water. Scrub the boulder with the soapy water, using a scrub brush. Spray the boulder with a garden hose to rinse the soap residue.
Spray the stone with stone cleaner, wiping off the cleaner as you go with a towel. Leave the boulder to dry.
Equip a polisher-grinder with a 50-grit diamond-polishing pad for granite. Separate types of polishing pads are available for white stone and for black stone.
Mist the first area to polish lightly with water. Start the polisher-grinder and hold the spinning pad flat against the stone's surface. Skate the pad slowly up and down and around the boulder. Stop and spray more water, as needed.
Switch to a 100-grit diamond-polishing pad and polish the stone again.
Continue to switch grits, moving through and re-polishing with 200, 400, 800, 1,500, 3,000 and 10,000-grit pads.
Wipe down the boulder with a damp towel.
Spray the boulder with granite sealer, wiping the surfaces of the granite lightly with a towel. Allow the sealer to dry, and then apply a second coat.