Synthetic ruby, which is single crystal aluminum oxide doped with chromium oxide, is being extensively used in several solid state devices, including microwave masers, coherent light oscillators (optical masers or lasers), and a new phonon-type acoustical maser. This paper describes the physical properties of synthetic ruby which are pertinent to its use in solid state applications.
Robert D. Olt, "Synthetic Maser Ruby," Appl. Opt. 1, 25-32 (1962)