Realgar and orpiment are both arsenic sulfides.  Realgar is an intensely reddish-orangish arsenic sulfide (AsS), while orpiment is a bright yellow-colored arsenic sulfide (As2S3).  They are always associated with each other.  Arsenic is a rare element in Earth's crust, but because As has very few uses in modern society, it has practically no value.  Orpiment & realgar have a nonmetallic luster and are fairly soft (H=1 for yellow orpiment & H=2 for reddish-orange realgar).  The two minerals are fairly insoluble, but they do volatilize readily.  When heated, they release a garlic smell (arsenic).  Realgar tends to alter to orpiment when exposed at Earth's surface.


Orpiment mass (yellow) with a little realgar (red) (6.8 cm across) from near Manhattan, Nevada, USA.



Realgar crystals (reddish-orange) with a little pararealgar (AsS, orangish-yellow) and a little orpiment (yellow) (field of view ~2.6 cm across).



Photo gallery of realgar

Photo gallery of orpiment



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