CAUTION: Before doing anything, visually inspect the inside of the geode and check for microscopic minerals, for example, goethite, millerite, or kaolinite. These are very tiny and easy to overlook. Some small crystal inclusions will be damaged by scrubbing and/or chemicals. Smythite, siderite, goethite, millerite, and strontianite are a few hard-to-come-by crystals that are most times destroyed by the rockhound when cleaning the specimen with a brush and water. If the geode, when opened, has iron oxide stain (brown mud coloring), these crystals in all probability have already been destroyed.
EASY METHOD: Simply wash the geodes in plain water with a bit of laundry detergent (or dish soap), then let them soak in a tub of water with 1/4 cup of ordinary household bleach for two days. This cleans most of the heavy grit off the geodes. Finally, clean the geode with a regular or electric toothbrush and ordinary denture cleaner, carefully covering all the crevices with the denture cleaner and brushing until all the grit is removed and the crystals are clean and bright.
WOOD BLEACH (OXALIC ACID) This method is preferred if there is iron staining in your geode. First, rinse the geode using a high pressure spray, removing all mud and dirt. Make a solution of oxalic acid, 1/8-cup oxalic acid per 5 liters of water is suggested. Soak the geode in wood bleach; at first, only soak for a few hours to avoid any damage to the geode. If the geode is still stained after the initial bath, soak longer or increase the concentration. When the geode is finished soaking, wash thoroughly with warm water. Add baking soda to the wood bleach before disposing to neutralize the solution.
CAUTION: Wood bleach can harm calcite crystals and carbonates. If you know or think you have either, you can brush on some diluted wood bleach solution, and immediately after the geode is clean wash and soak with warm water.
WARNING: Wood bleach is toxic! Wear eye protection, gloves, and a NIOSH-approved respirator mask to avoid injury. Do not use wood bleach in metal containers; it is corrosive and will eat through metal.