SIR C. V. RAMAN has recently1 made the suggestion that four different crystal structures of diamond exist, in which the carbon atom positions are similar, but in which the orientations of the tetrahedral carbon atoms in the two interpenetrating face-centred lattices differ. Two of these structures are tetrahedral, two octahedral; Raman suggests that type I diamonds consist of either of the tetrahedral forms, or of both interpenetrating (the deviation from the 'ideal' crystal increasing with interpenetration), and that type II diamonds consist of either or both of the octahedral forms; mixtures of tetrahedral and octahedral forms, he suggests, may also exist.