Comparative Investigation into the Appearance of Lapis Lazuli and Royal Blue

This paper examines the degrees of similarity between the appearance of the blue of lapis lazuli and the colour known as Royal Blue. Our hypothesis is that there is likely to be a resemblance in the appearance of both colours. There is an historic association of lapis lazuli blues with the depiction in Christian culture of both heavenly and earthly royalties hence the later use of the term Royal Blue. The colour of Lapis lazuli is intense blues of varying shades, thus it is to be expected that the term Royal Blue is likely to refer to a group of varying shades and not to a more or less precisely defined appearance. To test the hypothesis, we compared the appearance of Afghan lapis lazuli with samples of different art materials by different manufacturers that all bear the name Royal Blue. This allowed us to examine to what extent the appearance was kept stable across materials and manufacturers. Comparison of the results was carried out in CIELAB space as it describes the colours visible to the human eye and is device-independent. The relative perceptual difference between the colour of each sample of Royal Blue and the lapis lazuli sample was assessed by examining the Euclidean distance between them.