Orpiment is an arsenic-bearing sulfide group mineral (As2S3). It is known as a yellow mineral although the color may vary between darker (brownish) and brighter tones.
A sample from Zirneihbad, Kurdistan (Iran). Width of sample 9 cm. TUG 1608-1830.
The yellow color makes orpiment often similar to sulfur and these minerals may occur together in volcanically active areas, but orpiment has good cleavage (crystals are leafy) which may help to identify it if crystals are large enough to be seen. Orpiment is soft (Mohs hardness 1.5-2) and can be cut with a knife. Cleavage laminae are flexible but not elastic.
It came as a surprise to me that both Webmineral and Wikipedia claim that orpiment is a common mineral. I have hard time accepting that because orpiment contains arsenic which is not a common chemical element in the crust. Arsenic makes up only 1.8 ppm of the crust (which makes it less abundant than europium, for example) and orpiment is not the only mineral containing arsenic. Arsenic may occur in native form and realgar and arsenopyrite are well-known arsenic-bearing minerals as well. More than 100 minerals contain arsenic but most of them are exceedingly rare. Hence, I would say that orpiment as a common mineral seems to be an exaggeration. Whether it is rare depends on what is the definition of rareness.
What is the cause of such a confusion? I think the main reason is that it is a quite well-known mineral and has been pretty famous or maybe I should say infamous for a very long time. Golden yellow color of this mineral tricked alchemists to believe that it contains gold. Another reason to become famous is the fact that it is very toxic (because it contains arsenic). Inorganic arsenic is especially hazardous. It has been used to kill countless people. Orpiment (and other arsenic compounds) used to be very good poisons because it was difficult to say what exactly was the cause of death. Nowadays it is not the case anymore, arsenic poisoning is easy to detect. However, arsenic poisoning remains to be a major environmental and health-related concern. Drinking water is allowed to contain only up to 5 ppb or arsenic. Groundwater in many regions contains much more. It is estimated that well over 100 million people live in areas where groundwater contains too much arsenic to be considered safe to drink. Plants also pick up arsenic, but in organic compounds arsenic is not nearly as dangerous.
Orpiment is usually associated with realgar (AsS). These minerals precipitate out of volcanic gases or from hot springs and low temperature hydrothermal solutions.
The name “orpiment” is derived from Latin (auripigmentum) which means “golden paint”.