Many glass materials can be made more resistant to failure if the surface layers are in a state of permanent compressive stress. For purposes of monitoring production quality at the factory, as well as to verify compliance with specifications before incorporation into special devices, it is desirable to measure nondestructively this stress magnitude. For those glass compositions in which the stress generating process creates a residual, finely divided, and uniformly dispersed crystalline phase (e.g. “Pyroceram“), a stress measuring technique based upon X-ray diffraction has been adapted and successfully used. Lattice spacings for near-surface crystallites are determined under the stressed conditions present in the final glass product. These lattice spacings are compared with values obtained from a powdered sample of the identical glass composition. In the powdered state the induced stress effects are minimal. Hence, the shift in lattice spacings is a measure of the compressive surface stress level attained in the manufacturing process.