Alexandrite is named after Russian Tsar Alexander II (1818-1881), the very first crystals discovered in April 1834 near the Tokovaya River in the Urals, on the day the future tsar came of age. Although alexandrite is a relatively young gemstone, it is a fascinating gem since it shows both red and green.
Beautiful alexandrite in top quality, however, is very rare and hardly used in jewellery due to the high price. Its main feature is its ability to change its colour. Green in daylight, to a soft shade of red, in incandescent light. This unique optical characteristic makes it a highly prized - among the most expensive gemstones in the world, It is basically a chrysoberyl, but the presence of chromium accounts for the spectacular colour change.
Color Change in Alexandrite
Alexandrite is a trichroic gemstone which may absorb and reflect light differently in each of its three optical directions. The color change phenomena is a result of the presence of chromium +3 ions and the way they are absorbed and reflected. In alexandrite, the band is at 580nm. When the light is balanced (daylight), the stone will be green but when the light source is reddish (incandescent), the stone appears red.
Alexandrite or Chrysoberyl
In practice, chrysoberyl and alexandrite may be difficult to differentiate. Since stones with a weak change may be called alexandrite and stones with a faint change are called chrysoberyl, what is the difference? Where is the borderline?
Since the origin of the color change in alexandrite is due to the presence of chromium while the color of yellow or brown chrysoberyl is due to the presence of iron, a close examination of the spectroscopy will reveal the differences as both chromium and iron display characteristic spectrums. The yellow variety shows a broad band centered at about 4450 A in the blue violet while the alexandrite variety shows a chromium spectrum. The presence of chromium lines in the red orange area of the spectrum are diagnostic for alexandrite.
Fluorescence helps us to differentiate. Chrysoberyl owes its yellowish color to iron and usually shows no fluorescence. The red fluorescence of alexandrite can be observed using the “crossed filter” method and it has been inferred that in border line cases the presence of a red glow and a faint chromium spectrum absorption spectrum would prove the stone to be alexandrite.
