Orpiment is an orange to yellow mineral that is found worldwide, and occurs as a sublimation product in volcanic fumaroles, low temperature hydrothermal veins, hot springs and as a byproduct of the decay of another arsenic mineral, realgar. It is often found in association with realgar. It takes its name from the Latin auripigmentum (aurum - gold + pigmentum - pigment) because of its deep yellow color.
Sugested Reading: Pankaj Oudhia's Article on Hartaal.