The Collaborative International Dictionary of English v.0.48:

Realgar \Re*al"gar\ (r[-e]*[a^]l"g[~e]r), n. [F. r['e]algar, Sp.
   rejalgar, Ar. rahj al gh[=a]r powder of the mine.] (Min.)
   Arsenic sulphide, a mineral of a brilliant red color; red
   orpiment. It is also an artificial product.
   [1913 Webster] re-align

WordNet (r) 3.0 (2006):

realgar
    n 1: a rare soft orange mineral consisting of arsenic sulphide;
         an important ore of arsenic