vitreous: [17] Latin vitrum meant ‘glass’ (it may be the same word as vitrum ‘woad, plant producing blue dye’, the link being the bluishgreen colour of glass, and it might even be distantly related to English woad itself). From it was derived vitreus ‘clear, transparent’, which gave English vitreous. The sulphates of various metals have a glassy appearance, and so in medieval Latin the term vitriolum (a derivative of vitrum) was applied to them – whence English vitriol [14]. => vitriol[vitreous etymology, vitreous origin, 英语词源]