Realgar is a brilliant red stone found as small clusters in igneous extrusive layers. It is neither fire-safe nor magma-safe.
In real life
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.