A geode is a spherical stone with hollow spaces and crystal formations in its center. They are generally cut into two half-spheres to reveal the crystals inside. They may also be cut into slices or other shapes. Geodes are formed deep in animal burrows, beneath the roots of trees or in volcanic rock. The outer shell of a geode is plain and reveals nothing of the beautiful, intricate display of crystals on the inside, which were formed by thousands of years of minerals cooling and heating under pressure. Mud and other debris can cling to the outside of a geode and are easy to remove, but mud on the inside can stain the brilliant gem-like crystals geode hunters love to find.
Mix approximately 1 tbsp. laundry or dish soap with 1 gallon of warm water. Scrub the geodes gently with soapy water and a cloth to remove mud and debris. Rinse the geodes with clean water.
Add 1/4 cup of bleach to 1 gallon of water in a household bucket. Place the geodes in the bleach solution. Pour in additional bleach solution, if necessary, to completely submerge the geodes. Allow the stones to soak in the bucket for approximately 48 hours.
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Remove the geodes from the bucket. Apply a small amount of denture cleaner to a soft toothbrush. Scrub the geodes thoroughly with the toothbrush and cleanser. Clean all of the surfaces and inside crevasses. Gently remove any mud that remains inside the geode. If brown stains remain inside the geode after gently cleaning, do not vigorously scrub the stains, as you may damage the crystals. Rinse the geodes in clean, warm water to remove the cleaning product and the remaining grit you have loosened. Place the stones on a towel to dry.
If reddish-brown stains remain inside the geode after cleaning, they may be iron stains. Iron stains can be removed from geodes by cleaning them with oxalic acid; however, oxalic acid is toxic. Proper protective gear such as goggles, gloves and a respirator should be worn and other safety precautions taken if you choose to clean your geodes with oxalic acid.
The Inland Lapidary website reports that some crystals inside the geode can be destroyed by cleaning them with a brush. It is advisable to use the least-intrusive cleaning process first and seek advice from a lapidary expert before giving your geode more than a gentle cleaning.