Wipe the inside of a sink clean with paper towels. Lay an old towel flat on the countertop next to the sink.
Cover the drain with a drain stopper. Fill the sink with warm water, and add several drops of gentle dish soap.
Mist the decorative rock with undiluted white vinegar to remove mold, mildew or algae. Scrub the fungus off the rock with a scrub brush. Skip this step if fungus is not present on the rocks.
Submerge the decorative rock in the soapy water. Scrub the rock clean with a soft-bristled scrub brush.
Rinse the soapy solution off of the decorative rock with warm water. Place the clean rock on the towel. Allow it to dry for several hours.
Repeat Steps 3 through 5 until you have cleaned all of the decorative rocks.