The crystallization of laser glasses in the system (Y 2 O 3 ) 0.2 {((Al 2 O 3 ) x )(B 2 O 3 ) 0.8−x } (0.15≤x≤0.40) doped with 0.5mol% of ytterbium oxide has been investigated by x-ray powder diffraction, and various solid state NMR techniques. The crystallization process has been analyzed in a quantitative fashion by high-resolution solid state 11 B, 27 Al, and 89 Y NMR spectroscopy as well as 11 B{ 27 Al} and 27 Al{ 11 B} rotational echo double resonance (REDOR) experiments. The homogeneous glasses undergo major phase segregation processes resulting in crystalline Al 5 BO 9 (historically denoted as Al 18 B 4 O 33 ), YBO 3 , crystalline YAl 3 (BO 3 ) 4 , residual glassy B 2 O 3 , and an additional yet not identified crystalline phase (“X-phase”).