Study on Colour-Change Effect of Fluorite from Fuping, Hebei Province

A rare and precious species of gemstone—colour-change fluorite—has been discovered in Fuping, Hebei Province. The colour-change effect is described qualitatively and quantitatively with CIE colour system. The effect is greyish blue in the sunlight and reddish purple in the incandescent lamp light. Its chroma coordinates are (^x=0.3250, ^y=0.3313) in the standard D_ 65 illuminant and (^x=0.4717, ^y=0.3895) in the standard A illuminant. The colour-change degree is appraised quantitatively by calculating the chromatic aberration(△E^*). The big △E^* and hue angle difference(△H^*) in the different illuminants suggest that the colour-change effect is very distinct. Besides, the formation mechanism of the colour-change effect is explained with the visible absorption spectrum and the theory of modern colour psychology. The colour-change fluorite has the different transmission intensity in each wave band and the three cone cells of human eyes are stimulated differently in the different illuminant, which result in the different colour senses.