thumb|upright|Single-crystal sapphire boule grown by the Kyropoulos method. Approximately in diameter, weighing approximately . (A second boule is visible in the background.) Some sapphire-glass windows are made from pure sapphire boules that have been grown in a specific crystal orientation, typically along the optical axis, the c axis, for minimum birefringence for the application.
thumb|upright|Single-crystal sapphire boule grown by the Kyropoulos method. Approximately in diameter, weighing approximately . (A second boule is visible in the background.) Some sapphire-glass windows are made from pure sapphire boules that have been grown in a specific crystal orientation, typically along the optical axis, the c axis, for minimum birefringence for the application.