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.

Blue, green, and red light are defined by single, specific wavelengths of the electromagnetic spectrum (~470 nm, ~540 nm, and ~650 nm, respectively). In contrast, white light is not a single wavelength. It is the sum of multiple, distinct portions of the visible spectrum. Just as many different numbers can be added together to reach 100 (50 + 50, 33 + 67, 99 + 1, etc.) there are many ways to add colors of “pure” light to make white light.

One of the most common man-made sources of white light is black-body radiation. Metallurgist produce white light via black-body radiation when they heat metal in a furnace. Similarly, incandescent bulbs generate their glow by passing current across a metal element until it heats up and radiates. Yet, while black-body radiation is an effective means of producing white light, it is very energetically inefficient (most of the energy input is used to produce infrared light/heat). A much more efficient means of creating white light is to combine a few specific wavelengths of the visible spectrum. The color combinations that produce white light are depicted in the CIE color diagram below.

The colors along the rounded edge of the shape (everything but the bottom, straight edge) can be thought of as “pure” because they’re defined by a specific wavelength of light between 380 and 700 nm. All colors inside the border as well as along the bottom, straight edge are created when two or more “pure” colors are combined. White light is at the “center” of the CIE diagram (x = 0.33 and y = 0.33).

I regularly referenced this diagram while researching organic light emitting diodes (OLEDs) because molecules emit specific wavelengths of light and are not broad emitters (like heated metal). To make an OLED TV that displays most CIE colors, including white, manufactures incorporate blue (x = 0.1666, y = 0.0089), green (x = 0.2738, y = 0.7174) and red (x = 0.7347, y = 0.2653) emitting molecules in the screen design. To make an OLED screen appear yellow, both the red and the green molecules must be electronically excited and emit at the same time. The resulting color is entirely dependent on the proportion of red and green molecules excited. Exciting more green than red molecules makes the screen appear greenish-yellow. Exciting more red than green molecules makes it appear reddish-orange. If we want the screen to appear yellow, then the intensities of the emitting red and green molecules must be balanced. These “summed” emission can be depicted by drawing a straight line between the red and green points on the CIE diagram (image below).

Similar strategies are used to generate different types of white emitting OLEDs. Every day, ambient white light is sometimes created by summing the emissions of blue and yellow emitting molecules (image below left). White pixels on OLED TVs are created by summing red, green and blue emitters (image below right).

We perceive any light source emitting these two color combinations as white. However, illuminating an object, like alexandrite, under these various white light sources can uncover really interesting color chemistry.

Chrysoberyl is an oxide with the formula BeAl 2 O 4 which is typically colorless or yellow because it absorbs little to no visible light. Alexandrite is the rarest and most valuable member of the chrysoberyl family and is formed when some of the Al3+ is replaced by Cr3+, either naturally or intentionally.

The small amount of Cr3+ impurity in Alexandrite (<1 %) is directly responsible for its interesting colors. This coloration can be depicted via the absorption spectrum below.

This absorption spectrum is a graphical depiction of the amount of light absorbed/removed/not transmitted (y-axis) versus the wavelength of light (x-axis). Unlike undoped chrysoberyl, Alexandrite has two strong absorption peaks in the visible spectrum at ~400 nm and ~600 nm (for those of you crystal field junkies, the peaks at ~400 nm and ~600 nm are assigned to the 4A 2 to 4T 1 and the 4A 2 to 4T 2 transitions of octahedrally coordinated Cr3+). Conversely, it has two low absorptions, or high transmission windows, in the blue-green (470-520 nm) and red (>650 nm) portions of the spectrum.

When Alexandrite is viewed under uniform white light (the sum of ALL visible wavelengths of light) the blue and yellow portions of visible light are absorbed and the blue-green and red portions are not (below left). This gives the gem a purple-grey–the sum of blue-green and red emission–appearance (below right).

But, as I said above, not all white light sources are the same. Even though sunlight appears white if you look directly at it (don’t look directly at it!), it actually has a larger contribution from the blue and green portions of the spectrum. Under sunlight, Alexandrite absorbs yellow and blue. Yet, since more green and blue light is transmitted than red light, the gem appears blue-green, as depicted below.

In contrast, when Alexandrite is placed under incandescent lights or a candle, which have a larger contribution from the red portion of the spectrum, the gem appears red.

Based on the absorption spectra above, the Alexandrite effect could be greatly amplified if we viewed it under a two-component, white OLED (or a comparable two-color emitter). We could produce a white OLED by combining light from a ~490 nm and a ~590 nm emitter. When viewed under this light source, I’d expect Alexandrite to be a very sharp cyan color because the amber component (590 nm) would be entirely absorbed.

The relatively narrow emission of molecular species (50-100 nm) would also likely result in a much sharper color for Alexandrite than what is observed in the sun or under incandescent lights. If anyone has an easy way to perform this experiment, I would love to see the result.

That concludes my lengthy but thorough explanation of the interesting color chemistry of Alexandrite, a gem that is sometimes describes as an “emerald by day, ruby by night.” And maybe, experiment pending, this phrase will one day include “cyan by OLED.”

References

Farrell, E. F.; Newnham, R. E.; The American Mineralogist, 1965, 50, 1972-1981.

Liu, Y.; Shigley, J. E.; Fritsch, E.; Hemphill, S.; Mineralogical Magazine 1995, 59, 111-114.

http://www.alexandrite.net/chapters/chapter5/index.html

http://nature.berkeley.edu/classes/eps2//wisc/Lect7.html