Sapphire is an anisotropic, rhombohedral crystal form of Aluminum Oxide (Al2O3). In anisotropic single crystal materials, properties like thermal expansion and hardness can vary significantly depending on orientation. Sapphire also exhibits birefringence, an optical property that causes wave transmission to offset by up to 0.8% at right angles to the optic axis. This birefringence is eliminated along the crystal’s optic or C-axis. Therefore, for certain optical applications, specifying C-axis sapphire is crucial to avoid this effect.

Industrial sapphire is produced by melting aluminum oxide at 2040°C and promoting crystal growth using a seed crystal under carefully controlled conditions. Various growth methods have been developed, each offering different levels of quality, size, and cost. The EFG (Edge-defined Film-fed Growth) or Stepanov methods allow the growth of shapes like ribbons or tubes but come with many limitations. The Czochralski, HEM (Heat Exchanger Method), or Kyropoulos methods produce the
highest optical quality sapphire. However, these methods result in a rod-like “boule” that must be machined into usable shapes and sizes.

Sapphire and ruby are essentially the same material, with the only difference being the addition of small amounts of chromium (typically ≤ 0.05% by weight) in ruby. This addition affects colour and optical properties without significantly impacting the mechanical, thermal, and electrical properties.