Color Change in Alexandrite's

Alexandrite's are remarkable and rare gemstones. They display an extraordinary color change according to the ambient lighting, from emerald green in daylight to ruby red in incandescent light from tungsten lamps or candles. While this color change has been correctly attributed to chromium impurities and their absorption band in the yellow region of the visible light spectrum, no adequate explanation of the mechanism has been given. 

How Does Alexandrite Change Color?

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. 

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.

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. 

It is rare to find Alexandrite in large sizes. The specimens weighing more than three carats are not very common. The largest Alexandrite discovered so far weight around 66 carats and is located at Smithsonian Institution in Washington DC. Alexandrite is normally cut into conventional shapes like pear, oval, round and marquise.