剛玉化學式Al2O3是為無色。母岩地質環境孕育出不同顏色的剛玉，其顏色與內含微量化學成分有關。在紅寶石/藍寶石的顏色和微量元素之間的關係並沒有被完全解釋。此外為了比較與研究寶石的『顏色』，必須有科學化定義與量化方法來達成。此研究的目的是為了找到微量元素濃度和顏色的關係，並且希望能建立標準流程，能同時實踐於學術和商業領域。
在此研究中，我們收集不同產地語言色的剛玉樣品。其顏色依據 Gem and Jewelry Institute of Thailand–Gem Testing Laboratory（GIT-GTL）制定的顏色分級流程(Atichat et al., 2006) 將樣品顏色定義在CIE 1976 L*U * V *色彩空間，最後以亮度與色相來表示，以達到精確和科學化表述的顏色。通過紫外 - 可見 - 近紅外分光光度計獲得的樣品的光穿透反射光譜。微量元素的濃度通過雷射剝蝕感應耦合電漿質譜分析儀測定。
鉻、鐵、鈦、釩影響紅寶石和藍寶石的顏色。可見光透射光譜能夠揭示對顏色的微量元素的影響。鉻是紅寶石的紅色調的首要貢獻者，鈦在鉻濃度低（<1500ppm）時能增強紅色調，釩和鐵含量達到數百ppm便能使紅寶石的色調轉紫。 鈦鐵比能一窺藍寶石顏色：小於0.2時藍寶石色調為綠色或黃色，亮度也較高；當其接近或大於1時亮度降到極低。

The pure corundum, which comprises aluminium oxide, is colourless. The geological environment contributes different substance in the process of forming crystal so that the resultant corundum is in different colour. The relation between the colour and trace-elements in the ruby/sapphire was not fully explained. Also, a universal numerical system to defined colour is needed so that the colour of the gem could be state and compare with. The purpose of the study is to find the relation of trace element concentration and the colour, and also to establish a standard procedure which is practical in both academic and commercial field.
In this study we collected corundum samples vary in geological origin and colour. A colour grading system, which was developed by the Gem and Jewelry Institute of Thailand–Gem Testing Laboratory (GIT-GTL), was employed to identify the colour of samples (Atichat et al., 2006). On the basis of grading results, the colour of samples were defined in CIE 1976 L* u* v* colour space. The lightness and the hue of each sample was defined. The numerical expression of colour component described the colour precisely and scientifically. The optical transmission/reflection spectrum of samples were obtained via UV-Visible-NIR Spectrophotometers. The concentration of trace elements were measured via laser ablation-inductively coupled plasma-mass spectrometry.
Cr, Fe, Ti, and V affected the colour of ruby and sapphire. Visible light transmission spectrum was able to reveal the effect of trace elements on colour. Cr is the premier contributor of the red hue in ruby, and Ti enhanced red hue when Cr concentration was low (<1500ppm). A few hundred to thousands of V and Fe shifted the hue of ruby from red to purple. Ti/Fe ratio is vital to sapphire colour: when it was smaller than 0.2 the hue shifted to green or even yellow and the lightness was high; when it was much larger than 1 the lightness was too low to define the hue.

References
Atichat, W., Chandayor, P., Pisutha-Arnond, V., Wathanakul, P., Siripant, S., Saejoo, S., . . . Sutthirat, C. Ruby-Sapphire Quality Grading for the Gem Trade. Gems & Gemology, Vol. 42(Issue 3), 1. (2006).
Ball, S. H. A Historical Study of Precious Stone Valuations & Prices. Economic Geology, 30(5), 13. (1935).
Bristow, J. K., Tiana, D., Parker, S. C., & Walsh, A. Defect chemistry of Ti and Fe impurities and aggregates in Al2O3. Journal of Materials Chemistry A, 2(17), 6198-6208. (2014).
Burns, R. G. Mineralogical Applications of Crystal Field Theory: Cambridge University Press. (1993).
Burns, R. G., & Burns, V. M. Optical and Mossbauer spectra of transition metal-doped corundum and periclase. In Structure and properties of MgO and Al2O3 ceramics: American Ceramic Society. (1984).
Ferguson, J., & Fielding, P. E. The origins of the colours of yellow, green and blue sapphires. Chemical Physics Letters, 10(3), 262-265. (1971).
Germer, T. A., Zwinkels, J. C., & Tsai, B. K. Spectrophotometry: Accurate Measurement of Optical Properties of Materials: Elsevier Science. (2014).
Guo, Y., Li, X., & Tian, L. (2011). Cr3+ Effect on the Correlation between Lightness Difference and Color Difference of Ruby Red. Paper presented at the Advanced Materials Research.
Hughes, R. W. Red Rain • Mozambique Ruby Pours into the Market. (2015).
Hunt, R. W. G., & Pointer, M. R. Colour Order Systems Measuring Colour (pp. 155-195): John Wiley & Sons, Ltd. (2011)
Kuehni, R. G. Color: An Introduction to Practice and Principles: Wiley. (2012).
Liu, Y., Lu, T., Mu, T., Chen, H., & Ke, J. Color measurement of a ruby. Color Research & Application, 38(5), 328-333. (2013).
Nassau, K. The physics and chemistry of color: the fifteen causes of color: Wiley. (2001).
Orgel, L. E. ION COMPRESSION AND THE COLOUR OF RUBY. Nature, 179(4574), 1348-1348. (1957).
Palanza, V., Chiodini, N., Galli, A., Lorenzi, R., Moretti, F., Paleari, A., & Spinolo, G. Updating of the Interpretation of the Optical Absorption and Emission of Verneuil Synthetic and Natural Metamorphic blue Sapphire: the role of V 2+, V 3+ and Cr 2+. IOP Conference Series: Materials Science and Engineering, 15, 012087 (012088 pp.)-012087 (012088 pp.). (2010).
Poole, C. P. OPTICAL SPECTRA + COLOR OF CHROMIUM CONTAINING SOLIDS. Journal of Physics and Chemistry of Solids, 25(11), 1169-&. (1964).
Pryce, M. H. L., & Runciman, W. A. THE ABSORPTION SPECTRUM OF VANADIUM CORUNDUM. Discussions of the Faraday Society(26), 34-&. (1958).
Schanda, J. CIE Colorimetry Colorimetry (pp. 25-78): John Wiley & Sons, Inc. (2007)
Tilley, R. J. D. Colour from Atoms and Ions Colour and the Optical Properties of Materials (pp. 247-308): John Wiley & Sons, Ltd. (2011)
Zaw, K., Sutherland, L., Yui, T.-F., Meffre, S., & Thu, K. Vanadium-rich ruby and sapphire within Mogok Gemfield, Myanmar: implications for gem color and genesis. Mineralium Deposita, 50(1), 25-39. (2015).