Realgar is a brilliant red stone found in small quantities within igneous extrusive layers. It is one of the few stones which will boil in magma. Its melting point is so low that it is not fire-safe.
 In the Real World
Realgar is very soft. Being an arsenic sulfide (like orpiment), it is also an extremely toxic substance. It was used in painting for its unique orange colour, but fell out of use because of the health hazards. It poses no health hazards to dwarves, however, who will readily (and safely) use it as a normal stone, including for the production of food containers and mechanisms.