borrowed from German Melanit, from Greek melan-, stem of mélās "black, dark" + German -it-ite entry 1 — more at melano-
Note:
Term apparently introduced by the German mineralogist Dietrich Ludwig Gustav Karsten (1768-1810) in Göttingesches Journal der Naturwissenschaft, Band 1, Heft 2 (1798), pp. 138-40.