Realgar occurs as a soft, brittle mineral of a striking deep red to orange color, typically found in the same ore deposits as orpiment and frequently mentioned alongside it in mineralogical and alchemical texts from antiquity onward. Its name derives from Arabic, rahj al-ghar, powder of the mine or powder of the cave, a name that traveled into Latin and other European languages largely unchanged, testifying to the substantial transmission of practical mineralogical vocabulary from Arabic sources into medieval European alchemy. Ground to powder, realgar served as a pigment valued for a warm red-orange unmatched by most other available minerals, used in painting and, more cautiously given its toxicity, in some traditional medicines and pest control preparations, since arsenic compounds were recognized, at least empirically, as effective against vermin.
Alchemists treated realgar as a companion substance to orpiment, both arsenic sulfides differing mainly in color and in the ratio of arsenic to sulfur, and both were investigated as potential sources of arsenic for further compound preparations. Because both minerals are genuinely toxic, historical accounts of their handling, while less cautious than modern safety standards would require, do show an awareness that these were dangerous materials, not simply attractive pigments.