What is colorchange?
There are a number of effects that account for a change in color in any material. There is the Alexandrite effect, Usambaru effect and many luminescence effects like fluorescence, triboluminiscence, chemoluminiscence, thermoluminescence, and phosphorescence. The Alexandrite effect together with fluorescence and phosphorescence are maybe the most well-known effects. One by one the effect is described and explained.
1. Alexandrite effect
Also known as colorchange effect or color shift effect. One can debate for a long time what would be the right term to use when watching an alexandrite change color when viewing it in daylight as a green stone to red incandescent light. This is the most remarkable change ever seen. This does not only happen in alexandrite, but in many stones: Sapphires, triphilyte, granates, apatite, monazite, etc etc. Some have a strong effect like with some alexandrites others have a very week effect that can hardly be seen.
The basis of the change in color lies in the color of the light that is used. Part of the light is absorbed and part is reflected. The color is depending on our perception, the absorption and the type of light used. Daylight has more blue in its spectrum than incandescent light. Alexandrite has a big absorption in the green part of the spectrum and reflection in the blue and the red part. If viewed in daylight the red portion of the spectrum is absorbed by the stone and the blue and green part is reflected to the observer. Therefore the stone appears green. If incandescent light is used this is more yellow. We as humans correct the color of things by our mind so the paper that looked white in sunlight still looks white in incandescent light. But in reality is a bit yellowish. Never the less there is less blue in the spectrum. So the reflected light also has less blue in it. So the reflection to the observer is less blue and the same amount of red. The result is that there is not enough blue reflected to mix the perceived color to green. So the perceived color is more reddish. The less blue is in the light the more red the stone appears.
To be added: example graphs of spectrum of various light sources, absorption spectrum of alexandrite, schematic representation of the text.