The impact of trace metal cations and absorbed water on colour transition of turquoise

Abstract

Thirty-five gem-quality turquoise samples with various colours were investigated using energy-dispersive X-ray fluorescence spectroscopy, ultraviolet–visible spectroscopy, Fourier-transform infrared spectroscopy and scanning electron microscopy. Sample chemical and spectral analyses indicate that Fe 3+ contributes to green hue of turquoise, whose absorption band exhibits a bathochromic shift from 426 to 428 nm with increasing V content in the solid-solution series turquoise-chalcosiderite. V 3+ enhances absorption in the blue and orange regions, and Cr 3+ increases absorption in the green region, both of which are responsible for the vivid greenish yellow in faustite. Substitutions of Al by medium-sized trivalent cations (primarily Fe 3+ and V 3+ ) enhance polarity of the phosphate group (PO 4 ) 3− , resulting in strong absorption in the infrared spectra for analogues of turquoise. The reflectivity ratio ( R OH ) of the double absorption peaks at 781 and 833 cm −1 allows faustite to be distinguished from turquoise and chalcosiderite, with a value greater than 1, while V-rich faustite only has a single absorption peak at 798 cm −1 . An increasing amount of absorbed water contributes to blue chroma in turquoise and has a negative effect on lightness based on the CIE 1976 L * a * b * colour system. Loose turquoise with a low specific gravity tends to display greater colour differences with a significant decrease in lightness.

The Effect of Munsell Neutral Value Scale on the Color of Yellow Jadeite and Comparison between AP and K-Means Clustering Color Grading Schemes

“…To better ensure the accuracy of grading the colored gemstones, it is necessary to determine the objective grading environment. The CIE1931 color space system was first introduced to the color quantification study in gemology, however, to solve the problem that this system is inconsistent with the human visual perception [25], the CIE1976 L * a * b * uniform color space was born and applied to the quantitative characterization of gems which have been used extensively, such as sapphire [26], peridot [14], jadeite [13,[27][28][29], tourmaline [30,31], alexandrite [32], amethyst [33], turquoise [34], chrysoprase [35], garnet [36], beryl [37], etc. Based on the CIE1976 color space, Tang et al and Jiang et al analyzed the influence of Munsell neutral grey backgrounds on peridot and chrysoprase, respectively, and concluded that the background with high lightness was highly applicable to help distinguish the color of gems [38,39].…”

Research on parameters optimization of digital imaging system in red–yellow jadeite color measurement

“…These methods are mainly based on the subjective evaluation of the color by the operator, so it takes a lot of time and money to train professional graders. UV–Vis spectrophotometers can provide accurate absorption spectra from which color calculations can be made 2 , 3 ; however, the complex conversion process has hindered their development in gemstone color measurement; hand-held spectrophotometers, as the mainstream gem color measurement tool at present, has been used to measure the color of jadeite green 4 , jadeite red 5 , 6 , turquoise 7 , peridot 8 , 9 , amethyst 10 , chrysoprase 11 , 12 , and extensive gemstone chromatography studies. Because of its convenient operation and reliable data, this tool has already been favored by researchers.…”

Colour Quality Evaluation of Bluish-Green Serpentinite Based on the CIECAM16 Model

“…For example, GIA has used the Munsell system to form a set of independent and mature colour classification methods, such as yellow, pink, blue, and Cape series diamonds colour grading [14][15][16][17]. Many scholars studied the colour of gemstones based on the CIE1976 L*a*b* uniform colour space system, including jadeite [18], peridot [19,20], garnet [21][22][23], tourmaline [24,25], amethyst [26], and turquoise [27].…”