Modern chemistry identifies Green Vitriol cleanly as iron(II) sulfate heptahydrate, FeSO4.7H2O, one of a family of hydrated sulfate salts that lose water in stages when heated and that are collectively called melanterite in their natural mineral form. The identification was never seriously contested; unlike many alchemical names, vitriol of a specified color mapped onto a definite modern compound once the concept of a metal cation combined with a fixed sulfate anion was understood in the eighteenth century, chiefly through the work of Torbern Bergman and other Swedish and French chemists who systematized salt nomenclature. What took longer to sort out was the word vitriol itself, which unqualified could mean any of several sulfate salts; Lawrence Principe and other historians of chemistry note that early modern authors were not always careful to specify which vitriol they meant, and undifferentiated mentions of vitriol in older texts cannot always be assigned to iron with confidence. The color qualifier, green for iron, blue for copper, white for zinc, is the detail that actually resolves the identification, and where a text omits it modern readers are working from probability rather than certainty.
The ferrous, iron(II), state is also chemically unstable in air, which is why old green vitriol crystals develop a rusty brown crust of oxidized iron(III) sulfate on standing, a transformation early chymists observed without the electron-transfer chemistry to explain it.