The alexandrite variety displays a color change (alexandrite effect) dependent upon the nature of ambient lighting. Alexandrite effect is the phenomenon of an observed color change from greenish to reddish with a change in source illumination.Alexandrite results from small scale replacement of aluminium by chromium ions in the crystal structure, which causes intense absorption of light over a narrow range of wavelengths in the yellow region (580 nm) of the visible light spectrum. Because human vision is more sensitive to light in the green spectrum and the red spectrum, alexandrite appears greenish in daylight where a full spectrum of visible light is present and reddish in incandescent light which emits less green and blue spectrum. This color change is independent of any change of hue with viewing direction through the crystal that would arise from pleochroism.
Alexandrite 5 carats (1,000 mg) and larger were traditionally thought to be found only in the Ural Mountains, but have since been found in larger sizes in Brazil. Other deposits are located in India (Andhra Pradesh), Madagascar, Tanzania and Sri Lanka. Alexandrite in sizes over three carats are very rare.
Due to its high rarity in the Gem Market it is considered to be a precious stone in value, sometimes it is valued even more than precious stones such as sapphires and rubies depending on the percentage of its color change
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