The fracture toughness and path of stoichiometric spinel (MgAl2O4) crystals were determined at 22 degrees C for key low-index planes by double cantilever beam, as well as fractography of flexure specimens failing from either machining or indentation flaws. These results are compared with other single and polycrystalline MgAl2O4 fracture toughness values measured by various techniques, as well as single crystal versus polycrystal results for other materials. Evaluation of experimental and theoretical results shows (1)the fracture toughness of the spinel {110} plane is only a limited amount (e.g. 6%) higher than for the {100} plane (similar to 1.2 MPa m(1/2)), (2) fractography of machining flaw fracture origins was the most effective source of K-IC results, and (3) caution must be used in applying fracture toughness techniques to single crystals. Cautions include accounting for possible effects of elastic anisotropy (especial ly for double cantilever beam and probably double torsion tests), the nature of failure-initiating flaws (especial ly for notch-beam tests), and the frequent lack of symmetric plastic deformation and fracture (especially for indentation techniques).