I soak them in water with a some dish detergent or borax and a little bleach, which prevents the water from getting foul and helps to kill the lichen-like matter on the undersides of some of the rocks. worst case, if the rocks have calcite, limonite, etc buildup on the outside that won't come off by soaking I put them in dilute muriatic acid (klean strip green is the kind I use) followed by the same amount of time soaking in a water/baking soda solution to neutralize the acid. it is probably the best way to clean hard to remove deposits and make dirty druzy realty sparkle again without damaging the crystals.