Color grading of Emerald green based CIE 1976 L * a

This paper illustrated how to grade the color of emerald on the basis of CIE1976 L*a*b* uniform color space. In order to test the stability of Color i5 color photometer, sample average color variation was calculated by utilizing CIELabcolor difference formula and the result revealed that it is reliable. Tone of emerald was classified through comparing GemDialogue colorimetric method and instrumental measurement; in addition, merits and demerits of GemDialogue colorimetric were also illustrated during this process. The result showed that GemDialogue colorimetric method has certain fuzziness and subjectivity in color grading and the corresponding suggestions are also supposed. The green tone of emerald was classified into 4 parts by combining sample tone measurements with the drifting of its UV visible spectrum peak (10nm) under uniform color space. Finally, the color of emerald was graded. Introduction Emerald is known as “champion of green gems” due to its attractive velvet viridis and rarity; and it is listed into the world four precious stones together with diamond, ruby, and sapphire. Emerald is the birthstone of May as well as the memorial stone for 55th wedding anniversaries and symbolizes hopeful spring, vitality, safety and happiness. Truly superior emerald is very rare and the principle grading element is color, followed by neatness, weight, cutting origin and so on. The charming green in emerald is because of chrome or vanadium element and there are some subtle differences among the tone of emerald produced in different areas;Whereas, the development of emerald market is blocked by the following aspects. Firstly, the emerald is very expensive with big grade differences, which cannot be distinguished by common consumers. Secondly, the phenomenon of artificial synthesis and optimizing process in emerald market spreads unchecked. Thirdly, there is no authorized and detailed evaluation criteria for the green color which is most featured in emerald; while color grade occupies 2/3 in quality ranking system of emerald. At present, visual test is the main way of grading emerald, such as using master stone or color card; however, the subjective factors in visual test undermine its objectivity, science and uniformity. What’s more, in the qualitative evaluation for the color of emerald, people always cite some abstract professional words such as ivy shimmer ink, viridis, and blue-green, which lack of specific professional standard and reference system. There was researches on emerald in the past. For example, Liangting(1998) compared the color of emerald produced in Yunnan with that from Columbia on the basis of their gemological characteristics. As natural emerald, synthetic emerald, and light green beryl's bsome similarities in terms of composition and appearance, comparison of the three gemstones were made to explore the color and composition difference of these gemstones. Besides, common gemological testing methods and modern testing technology like electron probe micro-analysis (EPMA) and LA-ICP-MS were adopted to analyze the girdle of Yunnan emerald. Qugang [1]utilized CIE1931XYZ color system to quantitatively analyze visible absorption spectrum of Yunnan emerald. Although he primarily analyzed the difference of visible absorption spectrum for various 2nd International Conference on Economics, Social Science, Arts, Education and Management Engineering (ESSAEME 2016) © 2016. The authors Published by Atlantis Press 818 green hues (location of absorption peak, width of absorption band and absorption strength), he did not do the systematic research. In the research, he pointed out that Cr, V and Fe all can make the emerald takes on green; and emerald tends to be yellowish green when the Cr/V value is bigger; while, it tends to be blue green when Cr/V value is smaller. Whereas, the relationship between the content of Cr and Fe and each green index was unclear. At the earlier period, the problem of controlled crystallization of emerald single crystals from a fluxed melt, its color characteristics and optic parameters were discussed. But these researches on color features and color-causing mechanism stalled at colorimetric quantitative description and qualitative analysis[2-9]. In contrast, the research on emerald abroad focused on mineral deposit and crystal[10-18]. Quality evaluation for emerald color arouses jewelry researchers’ increasing attention. Most of the evaluations adopt qualitative or half quantitative method; and there has yet unified standard up to now. It is necessary to have objective and quantitative evaluation for the color of emerald from the perspective of gemology; so mass experimental studies are required. According to previous research, this study takes colorimeter COLOR i5 as the tool and adopts GemDialogue system and ultraviolet --visible spectrum to make grade classification for emerald color on the basis of colorimetric. Moreover, the study primarily discusses the objective and quantitative evaluation of emerald color, which provides the foundation for getting rid of subjectivity and fuzziness during color assessment and for the realization of objective and quantitative evaluation and finally set the grading standards for emerald color. Sample& Experiment The experiment selected 61 emeralds as samples and numbered them to ensure the representative ness and universality of tested color. Photographed the samples of emerald under illuminant D65 and the number of pictures is more than 60. Firstly, test the color of emerald samples with Color i5 color photometer, analyze and summarize; then divide the green tone of emerald using GemDialogue system, record and analyze the information. Discussion on Results Firstly, select emerald samples numbering 38, 39, 46, 50 and 54 to do Color i5 color photometer stability test, the selected 5samples are with broad representative ness because of the striking differences in their size, thickness and color. Measure the selected 5samples separately with color photometer each 5minutes, gaining the value of L, a* and b* measured each time, and calculating its mean. Take the mean of L, a* and b* as the reference value, then calculate the aberration with actual metrical data each time and this reference value. For example, test the stability of Color i5 photometer for #39 samples, the value of L, a* and b* of #39 sample measured at first time with color photometer is 53.91, -31.13 and 16.15 respectively, and the mean of L*, a* and b* of #39 sample is 53.9, -31.07 and 15.51 respectively measured after 10 times; take the mean of the value of L, a* and b* of #39 sample as guide sample to calculate the aberration of this two with color difference formula of CIE L, a* and b*: △E= , and the value is 0.21.