On the hardness of boron (III) oxide
  Our findings show that the hardness of glass-like B2O3 is of about 1.5 GPa, while the hardness of the high-pressure phase is higher by a factor of 10 (16(5) GPa) and comparable with the hardness (16 GPa) of the WC-10% Co hard alloy. 
V.A. Mukhanov, O.O. Kurakevych, V.L. Solozhenko
cond-mat.mtrl-sci