orpiment
[L. auripigmentum; aurum, gold, and pigmentum.]
Sulphuret of arsenic, found native and then an ore of arsenic, or artificially composed. The native orpiment appears in yellow, brilliant and seemingly talcky masses of various sizes. The red orpiment is called realgar. It is more or less lively and transparent, and often crystallized in bright needles. In this form it is called ruby of arsenic.