Its an easy 3 step procedure-
1-First step is critical -the clay/mud must be washed off the crystals- a toothbrush is good if you have a small piece to clean. Larger pieces can be placed in the sun for a day then cooled down in the shade and then given a wash with the garden hose. Repeating the sun process will dry and crack the clay and make for an easy rinse with the hose. You can use a pressure washer as well if clay is hard to get off.
2- Removing the iron: If the crystal has a very light iron staining then a few days soaking in a weak oxalic acid solution will do the trick -covered bucket.
**If iron staining is heavy then you must “cook” the quartz in an acid solution
3-The most commonly used chemical for cleaning quartz is oxalic acid which may be purchased in a powder form. When mixed with water at a few ounces per gallon and then heated to just below a boil it is capable of removing all but the most stubborn iron stains. WARNING – fumes are toxic and very dangerous. Only do this outside away from children and wearing protective gear.
** A slow cooker or crock pot works well.
**If your specimens begins to grow a white powder as they dry, place them back in a clean crock pot, add water and a 1/3 a cup of baking soda, and cook overnight. This will neutralize the remaining acid as it comes out of the nooks and crannies of the specimens. If this does not work to get rid of the white powder problem, then you will need to cook them again in clean water with baking soda as a neutralizer.
I have had to clean small crystal clusters as many as 5 times before coming totally clean, have patience.
I have also used the product “Iron Out” as well. It is sold at places like Walmart and is used to get rid of rust stains in sinks and toilets.