Fracture toughness is a critical structural design parameter and an excellent metric to rank structural materials. It determines fracture strength by way of the flaws, both inherent and induced, and defines the endpoint of the slow crack growth (SCG) curve. The fracture toughnesses of glasses and structural and optical ceramics as measured by the techniques in ASTM C1421 are compared. When good metrology is employed, the results are very comparable with two exceptions: materials exhibiting crack growth resistance (R-curve) and those with low SCG exponents. For materials exhibiting R-curves, the result is a function of extension and can be minimized with short cracks. For materials with low SCG exponents, such as glasses, elimination of the corrosive media and/or increasing the stress intensity rate produces consistent results. A summary of values is given for optical materials and glasses. ASTM C1421 is applicable with some modifications.