Magazine article American Scientist

The Color of Alexandrite

Magazine article American Scientist

The Color of Alexandrite

Article excerpt

Alexandrite is a rare form of the mineral chrysoberyl, which has the chemical formula Al2Be04. Not content with being one color, it can display a whole range of hues, depending on the light falling on it. Deposits of alexandrite were first discovered in the 1830s, in the Ural Mountains in Russia. Mineralogist Count Lev Alekseevich Perovskii presented it to the future Tsar Alexander П on his 16th birthday and named it in his honor.

Alexandrite's color is a consequence of impurities present in its chemical structure. In some places where an aluminum ion should sit, a chromium ion can be found instead. These impurities account for less than 1 percent of the aluminum sites, but this amount is enough to give alexandrite its hues.

Chromium ions absorb visible light strongly in the dark blue and yellow regions of the spectrum. Sunlight doesn't have a uniform contribution from all colors in the spectrum; there's slightly more green and blue light than red. Because more green and blue light remain unabsorbed than red light, alexandrite appears blue-green. In incandescent light, candlelight, or any light that is what we'd call "warm," the red end of the spectrum makes a much greater contribution. There's also much less blue and green. Because chromium ions don't absorb much red light, this leads to the purplered coloration of alexandrites in these conditions.

It's even possible to get one more color from alexandrite. If an ultraviolet lamp is shone on an alexandrite, an intense, glowing red color is seen. …

Search by... Author
Show... All Results Primary Sources Peer-reviewed

Oops!

An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.