The results of some new and previously published data from identation hardness tests on natural and synthetic quartz as a function of temperature provide several interesting conclusions. Indentation hardness appears to be affected by the concentration of some cationic impurities but not by others. A previous study showed that quartz hardness could be affected by electrolytically injecting lithium, but for the six natural quartz samples used in this study, no correlation between structural hydroxyl content and hardness existed. Several natural crystals showed hardening caused by heat treatments at temperatures of 600 ¿C or higher for times of the order of several hours. Two synthetic crystals were tested, and samples from one showed a correlation of hardness with total hydroxyl content. The hardness of the weakest synthetic crystal was never less than 50% of the average of the natural quartz values at all temperatures up to 850 ¿C.