- Nickel‐doped yttrium aluminum garnet (Ni: YAG) has an optical absorption spectrum characteristic of Ni+ 3 located on octahedral sites with an absorption peak at 420 nm. When annealed in reducing atmospheres, the nickel converts to Ni+ 2 and the 420‐nm absorption peak vanishes. Codoping with zirconium fixes the nickel into the Ni+ 2 state under all annealing conditions. Under these circumstances, some of the nickel is believed to enter the lattice on tetrahedral sites. The characteristic 320‐nm defect emission is observed in all crystals.