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The alexandrite gem stone variety displays a color change (alexandrite effect) dependent upon the nature of ambient lighting. This color shift is independent of any change of tint with viewing direction through the crystal that would arise from pleochroism. Both these different properties are usually referred to as color change, however. Alexandrite results from small 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 of the spectrum. Alexandrite from the Ural Mountains in Russia is green by daylight and red by incandescent light. Some varieties of alexandrite may be yellowish or pink in daylight and have a red or raspberry red color by incandescent light.
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