Chromaticity Contrast Analysis of Several Common Blue Gemstones

Sapphire, apatite and cordierite are common blue gemstones that in some way similar, but there are still many differences in their colours. Subjective colour evaluation cannot accurately quantify the specific colour differences of the three gemstones. This study used the principle of chromaticity to measure the correlated chromaticity data of sapphire, apatite and cordierite. Quality analysis and evaluation were performed under the CIE1931 chromaticity system and CIE1976 La^*b^* colour space system. The colours of the three gemstones can be divided into four hues: blue, blue-violet, blue-cyan and blue-green. The results show that sapphire shows the bluest hue, followed by apatite. Sapphire shows more green hue, and apatite shows more yellow hue. It is concluded that the blue-green sapphires have the worst chroma, pure blue and blue-violet sapphires have the best colour quality; blue-cyan and blue-green apatites have the best colour quality. The lightness of cordierite has a certain negative correlation with the chroma: blue and blue-violet hues of cordierite have excellent chroma but poor lightness, while blue-cyan and blue-green hues of cordierite have good lightness but poor chroma. From this, the correspondence between the colour quality and chromaticity parameters of the three blue gemstones can be deduced, which provides a scientific theoretical basis for colour quality grading.