Gemstones with an unusual color change like effect

Abstract

The conventional color change in gemmology means the effect to change its color from green or blue green under day light (Illuminant D65) to red or purple red under the incandescent light (A illuminant). This mechanism is that such color change gemstones absorbs yellow light strongly and transmits green and red light contrastively. However there are some gemstones which change or seem to change their colors by other mechanism.
First, some gemstones change their color when strong light shines them from side. The pink sapphire from Batakundhi, Pakistan show blue color on the surface. This is one of the Rayleigh scattering. The micro inclusions scatter the blue light and it gives purplish tone to the stone. The similar effect can be seen with moonstone with blue schiller. In corundum, pink sapphire from Sri Lanka or ruby from Tajikistan also show some kind of phenomena. And this is also the beauty of Kashmir sapphire.
Second, color change with lighting with some direction can be seen with green andesine. Some andesine show green color with transmitted light, but is show red color on the surface with reflected light of same light source. When we measure the UV-visible spectra with transmission method and reflection method, the spectra differ as Figure.1 Such andesine has many micro inclusions of copper platelet. If the light transmits the stone, the copper platelet does not affect the color, only as a shadow. But if the light shines the stone, the light reflects from the copper platelet and cause orangish red color. Third, the recent scapolite from Afghanistan show very strong teneblescence. When the short wave UV is irradiated at this scapolite, the colorless stone changes its color to vivid violetish blue in 10 seconds. (Figure.2.) After it, it can be return to the original colorless color with strong light or heating in a few second. It also has reversibility.
As mentioned above, there are many gemstones change or seems to change its color not by conventional color change effect.