Abstract The behavior of brittle materials under biaxial and triaxial loading is predictable on the basis of stress envelopes derived from the properties of idealized materials. In the case of nonmetallic brittle materials, the idealized patterns must be somewhat modified to allow for the low tensile strength of these materials. Two basic criteria are recognized: one related to the angle of internal friction, the other a limiting-stress criterion (in tension). The paper culminates in the construction of three-dimensional stress envelopes by means of contours.