haematoxylin

A blue dye obtained from the leguminous tree logwood (Haematoxylon campechianum) that stains nuclei and cellulose cell walls blue. Haematoxylin itself is not a stain until it has been oxidized to haematin. It also usually requires a mordant, such as iron alum or an aluminium salt. Different haematoxylin solutions can be prepared,  e.g.  Delafield's haematoxylin, which can be used as a counterstain with safranin. See also staining.