Wiktionary
villiaumite
n. (context mineralogy English) A rare toxic halide mineral composed of sodium fluoride.
Wikipedia
Villiaumite
Villiaumite is a rare halide mineral composed of sodium fluoride, Na F. It is very soluble in water and some specimens fluoresce under long and short wave ultraviolet light. It has a Mohs hardness of 2.5 and is usually red, pink, or orange in color. It is toxic to humans.
The red color is due to a broad absorption peaking at 512 nm. It is a result of radiation damage to the crystal.