Rhinestones are the first to show age on your costume jewelry, if they have gotten wet and have foiled backs the water will destroy the foil, turn it black or dark and I don't know of a way to save them you will have to replace them, moisture is your rhinestones worse enemy.
I use a very soft brush to clean them first then I use a Q-tip and windex, since it dries quickly, you can also use rubbing alcohol in the same manner, spray the cleaner in a small bowl barely dampen the Q-tip and rub lightly over the top of the rhinestone, then use a soft cloth to dry it, turn the item upside down on a soft cloth so in case there is a drip it won't run under the foil.
