The rate of wear of a single crystal sapphire sphere depends on crystal orientation and slide direction. This effect can be correlated with the shear mechanism of the crystal structure at elevated temperatures. On oxidized metal surfaces or on glass, sapphire appears to wear more rapidly than on freshly generated metal surfaces as a result of chemical degradation. The effect of crystal orientation, which is associated with the shear strength of the crystal, is therefore decreased. Lubrication appears to decrease wear, but the effect of crystal orientation on wear rate persists.Presented as an American Society of Lubrication Engineers paper at the Lubrication Conference held in Rochester, New York, October 15–17, 1963.