Realgar,
a-As
4S
4, is an
arsenic sulfide mineral, also known as "ruby sulphur" or "ruby of arsenic". It is a soft,
sectile mineral occurring in
monoclinic crystals, or in granular, compact, or powdery form, often in association with the related mineral,
orpiment (As
2S
3). It is orange-red in colour, melts at 320 °C, and burns with a bluish flame releasing fumes of arsenic and sulfur. Realgar is soft with a
Mohs hardness of 1.5 to 2 and has a
specific gravity of 3.5. Its
streak is orange colored. It is
trimorphous with
alacranite and
pararealgar. Its name comes from the Arabic
rahj al-gar (, "powder of the mine"), via
Catalan and
Medieval Latin, and its earliest record in English is in the 1390s.