The color changing property of Alexandrite is known as ‘Alexandrite effect’ and is a result of chromium traces found in the gemstone. The chromium traces cause strong light absorption in the yellow and blue parts of the spectrum. The absorption band 580 nm allows the Alexandrite to change color from red to green when seen under different light.
The image above shows what makes up the different types of lighting we commonly see. It is clear to see that sunlight has a fairly flat range while Fluorescent lamps contain very light blue light and lots of yellow light.
When Alexandrite is exposed to these types of light, the gemstone absorbs certain wavelengths while reflecting others. Alexandrite is very good at reflecting green wavelengths of color, so when it is seen in daylight you can see there is plenty of green light to reflect (yellow line). However, when this gemstone is moved inside under a fluorescent lamp, you can see there is not much green light, but plenty of red light. The Alexandrite will try to reflect as much green light as possible, but since there is not much green light the gem will also be reflecting the large amount of red light (blue line)
In Sri Lanka, the Alexandrite exhibits a brown or khaki hue when it changes color. In Zimbabwe, the Alexandrite have a minor color change and normally reflect the darker shades of purple. Tanzanian Alexandrite also possess lighter tone and change color moderately. The Alexandrite in Brazil are highly saturated and exhibit a color change from blue to purplish tones. The most precious Alexandrite are those that contain pure shades and change colors greatly.