On the Hardness of a New Boron Phase, Orthorhombic {\gamma}-B28
  Measurements of the hardness of a new high-pressure boron phase, orthorhombic {\gamma}-B28, are reported. According to the data obtained, {\gamma}-B28 has the highest hardness (~50 GPa) of all known crystalline modifications of boron. 
V.L. Solozhenko, O.O. Kurakevych, A.R. Oganov
cond-mat.mtrl-sci