Arsenic reached the alchemists as two brilliantly coloured sulfide minerals. Orpiment is a golden yellow arsenic trisulfide, and its Latin name auripigmentum means gold pigment; realgar is a scarlet arsenic sulfide whose name comes into European languages through the Arabic rahj al-ghar, powder of the mine. Both were mined, ground and used as pigments across the ancient and medieval world, in Egyptian painting, in Chinese and Persian manuscripts and in European panel painting, and both are seriously poisonous.
Their importance in alchemy goes beyond colour. Orpiment's golden appearance made it an obvious candidate for a substance containing the principle of gold, and arsenic compounds do in fact whiten copper, producing an alloy that convincingly resembles silver. That effect is documented in early recipe collections, and it is one of the concrete practical results that kept transmutation credible: something was visibly happening to the metal.
Isolation of the element is usually credited to Albertus Magnus, around 1250, by heating orpiment with soap, although earlier preparations in the Arabic tradition are likely. Arsenic has a recognised alchemical symbol and appears throughout the corpus, but it was never one of the seven planetary metals and was never assigned a planet. Like antimony, bismuth and zinc, it is a substance the tradition knew well and could not fit into its own scheme.
Arsenic sublimes rather than melting at ordinary pressure, passing straight to vapour above about 600 degrees C, and the vapour smells of garlic. That odour was a recognised warning sign in mining and metalworking for centuries.