A distinctive form of cracking is seen to occur as a result of the thermal exopansion mismatch stresses created between a cylindrical sapphire rod and a borosilicate glass matrix cooled after bonding at high temperature. The cracks, having a resemblance to cone cracks formed on Hertzian contract, nucleate at the free surface of the matrix in the vicinity of the rod intersection with the surface. However, rather than form at the rod/matrix interface, the cracks nucleate at some distance away that is found to vary from one sample to another. In general, the cracks extend further into the matrix the closer they nucleate to the rod. In addition, the crack trajectories vary both in angle to the rod axis and in shape, sometimes being curved. The crack path is calculated to be dependent on the relative magnitudes of the axial and radial residual stresses in the matrix. The cracks did not form immediately after cooling but only after exposure to the laboratory air. In some cases cracking was initiated by mechanical action, such as sectioning. In addition to the cone cracks at the ends of the rod, cone-like cracks were occasionally also seen to form from internal fractures of the sapphire rod on cooling after fabrication. © 1990.