Gentian Violet (crystal violet, Methyl Violet 10B, hexamethyl pararosaniline chloride) is a bactericide and an anti-fungal agent. The primary agent used in the Gram stain test, Gentian Violet is perhaps the single most important bacterial identification test in use today. It is also used by hospitals for the treatment of serious heat burns and other injuries to the skin and gums.
Typically prepared as a weak (e.g. 1%) solution in water, it is painted on skin or gums to treat or prevent fungal infections. Gentian Violet does not require a doctor's prescription (in the US), but is not easily found in drug stores.
In body piercing, Gentian Violet is commonly used to mark the location for placing piercings, including surface piercings.