For example, Fluorite and Amethyst. Please be aware and do not leave them out for a long period of time. NOTE - Mr. Cunningham also has a full Wiccan Ritual for cleansing the stones. Her suggestions can be found on pages 54-55. This is quite an interesting idea, so I felt I should share it here.
Melody suggests soaking the crystal in brown rice.
Soak the crystal in brown rice for twenty-four hours, the rice balances and centers the energy, removing the negativity, while dissipating and transforming the negative to the positive. Upon completion of the 'soak' the rice is purified and energized and is quite wonderful to eat. The salt draws all the negativity out of the stones, and as salt is, itself, a crystal, I feel this is a very compatible method for cleansing crystals. I only bury them for 3 whole days and nights when they are brand new, and I have never cleansed them before. Within it's walls may love abound, and keep it safe, and keep it sound.