Another form of microscopy is fluorescent microscopy done by staining with auramine. Instead, acid-fast stains such as Ziehl-Neelsen stain, or fluorescent stains such as auramine are used. Auramine O is a diarylmethane dye used as a fluorescent stain. In its pure form, Auramine O appears as yellow needle crystals. The first dyes manufactured by them were alizarinblue and auramine. Auramine O, a dye, is a salt of the iminium cation [(CH)NCH]CNH.