[1] [1] Werfel F, Brummer O. Physica Scripta, 1983, 28: 92.

[2] [2] Emmett J L, et al. Gems and Gemology, 2003, 39(2): 84.

[3] [3] Nassau K. The Physics and Chemistry of Color: The Fifteen Causes of Color. Wiley, New York, 1983.

[4] [4] Ferguson J, Fielding P E. Chem. Phys. Lett., 1971, 10(3): 262.

[5] [5] Krebs J J, Maisch W G. Phys. Rev. B, 1971, 4(3): 757.

[6] [6] Mohapatra S K, Kroger F A. J. Am. Ceram. Soc., 1977, 60(3/4): 141.

[7] [7] Wang H A, et al. Physical Review B, 1983, 27(6): 3821.

[8] [8] Pichet L, et al. Chin. Phys. Let., 2008, 25(6): 1976.

[9] [9] Bedu-Amissah K, et al. J. Am. Ceram., 2007, 90(5): 1551.

[10] [10] Levinson A A, Cook F A. Gems & Gemology, 1994, 30(4): 253.

[11] [11] Figgis B N, Hitchman M A. Ligand Field Theory and Its Applications. Wiley-VCH, New York, 2000.

[12] [12] Kronenberg A K, et al. Acta Mater, 2011, 48(7): 1481.