|This 7.19-carat alexandrite was cut to feature its beautiful color change. When the light source changes from daylight to incandescent light, the gem’s color changes from bluish green to reddish purple.|
Alexandrite is the variety of chrysoberyl BeAl2O4 which changes color from green or blue-green in daylight to red, purple/red, or raspberry red under incandescent light. Chemically, alexandrite can be differentiated from normal chrysoberyl by the presence of chromium. Alexandrite is defined by its color change and only chrysoberyl that displays a distinct change of color should be referred to as alexandrite.
The origin of this color change is often attributed to dichroism, but simple observation shows that the change depends chiefly on the nature of the incident light. In antiquity this change was from sunlight to candlelight and this exhibition is still used today by many gemologists and jewelers.
Alexandrite is a gem that exhibits an amazing property. It appears red in incandescent light and green in sunlight. Incandescent light and sunlight both appear white when we look at them but, as Alexandrite demonstrates, not all white light is the same. Differences in white light sources can have a profound effect on how we perceive an object’s color. The Alexandrite Effect is a perfect example.
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 from the Ural Mountains in Russia can be green by daylight and red by incandescent light. Other varieties of alexandrite may be yellowish or pink in daylight and a columbine or raspberry red by incandescent light.
Stones that show a dramatic color change and strong colors (e.g. red-to-green) are rare and sought-after, but stones that show less distinct colors (e.g. yellowish green changing to brownish yellow) may also be considered alexandrite by gem labs such as the Gemological Institute of America.
Today, several labs can produce synthetic lab-grown stones with the same chemical and physical properties as natural alexandrite. Several methods can produce flux-grown alexandrite, Czocchralski (or pulled) alexandrite, and hydrothermally-produced alexandrite. Flux-grown gems that are fairly difficult to distinguish from natural alexandrite as they contain inclusions that can look natural.
Czochralski or pulled alexandrite is easier to identify because it is very clean and contains curved striations visible under magnification. Although the color change in pulled stones can be from blue to red, the color change does not truly resemble that of natural alexandrite from any deposit. Hydrothermal lab-grown alexandrite has identical physical and chemical properties to real alexandrite.
Some gemstones falsely described as Lab-grown synthetic alexandrite are actually corundum laced with trace elements (e.g., vanadium) or color-change spinel and are not actually chrysoberyl. As a result, they would be more accurately described as simulated alexandrite rather than synthetic. This alexandrite-like sapphire material has been around for almost 100 years and shows a characteristic purple-mauve colour change, which does not really look like alexandrite because there is never any green.
See also: This Gemstone Switches Colours in an Instant