Chiang Mai Journal of Science

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The colour of gem spinel is caused by various transition metals incorporated during the growth process of the crystal and act as impurities in the spinel structure. Various amounts, ratios and coordination sites of the transition metals produce different colours. Mogok spinel displays various colours such as red, violet, blue, green, yellow and colourless. UV-Vis-NIR spectroscopy can be used to study the mechanisms of coloration caused by transition metal in gemstones. This technique was applied to colourless, red, violet and greenish blue Mogok spinels. Colourless samples show no absorption feature in visible range. The crystal field transition (d-d) for Cr³⁺ (in octahedral coordination) bands centered at approximately 388, 415 and 540 nanometer dominate the absorption spectra of red Mogok spinels. Greenish blue and violet Mogok spinel are dominated by d-d Fe²⁺ (in tetrahedral coordination) peaks at 371, 383., 457, 559 and 659 nanometer, a broad absorption band centered at approximately 2000 nanometer and by Fe²⁺_tet/Fe³⁺_tet charge transfer narrow band at approximately 912 to 925 nanometer

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