The equation of state of boron has been measured up to 100 GPa by single-crystal x-ray diffraction with helium as the pressure transmitting medium. Rhombohedral beta-boron is the stable structure up to 100 GPa under hydrostatic conditions. Nonhydrostatic stress stabilizes a different rhombohedral structure. At about 100 GPa a pressure-induced amorphization is observed. The amorphous phase can be quenched to ambient pressure. An explanation is proposed based on the different stability under pressure between intraicosahedra and intericosahedra bonds.