Green vitriol, or sulphate of iron; a green crystalline substance, of an astringent taste, used in making ink, in dyeing black, as a tonic in medicine, etc. It is made on a large scale by the oxidation of iron pyrites. Called also ferrous sulphate.

iron (II) sulfate (FeSO4.7H2O, green vitriol)

a compound of iron or iron-sulphate, used in dyeing or tanning cloth, or making ink; normally green, blue, or blue-green