Alum stone is a natural cosmetic commonly used as a deodorant and aftershave. It is made from the naturally occurring mineral salt potassium alum. Potassium alum forms as encrustations on rocks near volcanic fumaroles. It appears as a translucent crystalline stone and is soluble in water. Alum stone has antimicrobial properties which allow it to prevent the growth of odor causing bacteria. It can also be used as an astringent and styptic.