Realgar

Specimen from The Proctor Gem Crystal and Mineral Family Trust Collection, via Bijoux et Mineraux. 

"Realgar, α-As4S4, is an arsenic sulfide mineral, also known as "ruby sulphur" or "ruby of arsenic"... name comes from the Arabic rahj al-ġār (رهج الغار, "powder of the mine"), via Catalan and Medieval Latin...

Realgar most commonly occurs as a low-temperature hydrothermal vein mineral associated with other arsenic and antimony minerals. It also occurs as volcanic sublimations and in hot spring deposits...

Realgar, orpiment, and arsenopyrite provide nearly all the world's supply of arsenic as a byproduct of smelting concentrates derived from these ores... Realgar is poisonous. The ancient Greeks, who called it "sandaracha," knew that it was poisonous. It was used to poison rats in medieval Spain and in 16th century England."