A crystal deodorant is a deodorant product made with potassium alum or ammonium alum. Both are double sulfate forms of the class of minerals known as alums. Alum crystals are highly soluble in water, which explains their ability to act as a deodorant.
When a crystal deodorant is applied to the underarms, some of the mineral is dissolved by the body's sweat (the stick can also be pre-moistened with water). This leaves behind a thin layer of mineral salts on the skin's surface. That layer of salt prevents body sweat from coming into contact with odor-causing bacteria.
Unlike antiperspirants, which reduce the amount of sweat the body produces, deodorants are designed to prevent naturally odorless sweat from becoming a breeding ground for bacteria. Most commercial deodorant products contain antibacterial agents or perfumes to prevent body odor or mask its scent. People who choose to use a crystal deodorant instead often do so because they are concerned about the potential health risks of applying such chemicals to the thin and highly absorbent layer of skin under the arm.
Many people also find that rock deodorants are gentler on the skin. The Mayo Clinic notes that commercial antiperspirants are linked to skin irritation more than any other cosmetic product.
Crystal deodorants have long been used in many cultures, including ancient Egypt, China and other parts of Asia. The Roman philosopher Pliny (AD 23 - 79) gave one of the first recorded descriptions of the properties of alum crystals in his encyclopedia Natural History.
Today, Thailand is one of the largest producers of commercial deodorant stones. The stones are manufactured rather than mined. Mineral salts taken from the Earth are crystallized from an aqueous solution then hand-shaped into crystal deodorant form before being packaged and shipped to market. Numerous companies sell these rock deodorants under a variety of product names.
In addition to the benefits of being natural, a crystal deodorant can also be a cost-effective alternative for preventing body odor. A single crystal deodorant stone can last up to one year -- much longer than the typical commercial deodorant product. Unlike other deodorants and antiperspirants, rock deodorants are also hypoallergenic, don't stain clothing and do not involve lab testing on animals.