Sapphire is an anisotropic, rhombohedral crystal, and therefore non-uniformly dimensioned. Anisotropic materials exhibit some properties such as thermal expansion uniquely to orientation. For most applications this is unimportant, however should be considered. Sapphire is “birefringent” which is a refractive feature that offsets wave transmission up to 0.8% at right angles to the optic axis. Birefringence is eliminated along the optic or C-axis. For certain applications, C-axis sapphire should therefore be specified to avoid birefringence concerns.