Realgar

Specimen from Heritage Auctions.

Its name comes from the Arabic rahj al-ġār (رهج الغار, "powder of the mine"), via Catalan and Medieval Latin, and its earliest record in English is in the 1390s...

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.

Another specimen here.