Orpiment is an unremarkable yellow stone found in small quantities in igneous extrusive layers. It is not fire-safe and belongs to the few stones which will boil in magma.
In Real Life
Orpiment is an arsenic sulfide (as is realgar), and is extremely toxic in real life. It poses no health hazards in Dwarf Fortress, however – dwarves can consume food and drink stored in orpiment pots without harm.
another arsenic sulfide, comes from realgar