Colour Analysis of Turquoise Using Uniform Chromaticity Coordinate

In recent years, due to the increasing market demand for turquoise, turquoise prices are rising. Therefore, grading the quality for turquoise is very necessary. Turquoise colour is the primary factor in the quality grading, so its classification is of primary importance. Currently in the jewelry market, only 4C diamond grading system is widely recognized by the industry; and such evaluation of turquoise colour still remains in the visual and textual description, lack of objectivity. In this papers, the authors chose 45 pieces of slightly impregated turquoises and 8 pieces of natural untreated turquoises from the market, with colour range of sky blue to green as studying materials. Using the calculation formula of colorimetry, the reflectance values of the samples were converted to a set of chromaticity coordinate values. Then, the authors analyzed the hue, saturation and brightness of the samples. It is found, in the sky blue to green series turquoise, the dominant wavelength mainly falls within the range of 480~500 nm, with low saturation and middle brightness. For the best blue colour sample, the dominant wavelength is at 480.3 nm, the saturation is 41%, and the brightness value is not higher than 65%.