The analysis of all the data (Tables and) allows us to conclude that the high density gives rise to high hardness values, which are the highest for the qtz allotropes mentioned above; but this does not allow us to say that their hardness is higher than that of diamond. Finally, the bulk moduli are also higher for the qtz C 3 , C 6 and C 12 allotropes, but remain within the limits of diamond/lonsdaleite.