Summary: Traces of a green chronian tourmaline occur in calcareous rocks in contact with serpentinite near Alpurai, Swat, West Pakistan. The petrography of the rocks and the optical properties of the tourmaline are presented along with its chemistry (it contains over 8 % Cr₂O₃ and about 0·2 % V₂O₃). The mineral is compared with other chromian tourmalines and its paragenesis discussed: it is thought to have been produced by silica-rich hydrothermal (or gaseous) solutions, which have crystallized as thin quartz veins in the calcareous rocks, introducing boron and other constituents.