Topic summary
Alexandrite
Extracted from the Wikipedia article Chrysoberyl.
Alexandrite
The alexandrite variety displays a color change dependent upon the nature of ambient lighting. Alexandrite results from small scale replacement of aluminum by chromium ions in the crystal structure, which causes intense absorption of light over a narrow range of wavelengths in the yellow region (520–620 nm) of the visible light spectrum. Because human vision is most sensitive to green light and least sensitive to red light, alexandrite appears greenish in daylight where the full spectrum of visible light is present, and reddish in incandescent light which emits less green and blue light. This color change is independent of any change of hue with viewing direction through the crystal that would arise from pleochroism.