Crystallographically oriented single-crystal prisms of gem quality topaz (composition Al2SiO4 (OHx-F1-x)2 where x=0.04¿0.01) were deformed at a confining pressure of 1.50 GPa, a temperature of 800 ¿C, and a strain rate of 2¿10-5 s-1. Under nearly identical conditions, all crystals of anhydrous rock-forming minerals that have been tested to date, such as olivine, quartz, feldspars, pyroxenes, and refractory oxides, deform plastically; in contrast, our topaz crystals failed by brittle fracture regardless of the orientation of the compression direction. No optical evidence for plastic deformation was detected. Another suite of experiments with compression perpendicular to the (001) cleavage at T=100¿--950 ¿C and a strain rate of 2¿10-5 s-1 displayed two regimes of behavior: (1) at T>400 ¿C, fracture strength was independent of temperature, and fracture occurred on one or two surfaces parallel to {103}:(2) at T<400 ¿C, the fracture strength increased rapidly with decreasing temperature, no macroscopic stress drop was observed, and many closely spaced conjugate fractures formed on (103) and (1¿03). The anomalous brittleness of topaz compared to anhydrous silicate and oxide crystals indicates that intracrystalline ''water'' plays a role in the embrittlement. We suggest that water within the topaz crystals promotes fracture in ways similar to the mechanisms of slow crack growth aided by environmental moisture.