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Alexandrite

Hardness : 8.5
Specific Gravity : 3.73
Refractive index : 1.741 - 1.

Alexandrite was discovered in 1830 on the birthday of Alexander II in the rural mountains in Russia. Alexandrite is also found in Sri Lanka, Brazil, India, Tanzania, Zimbabwe, and most recently in Madagascar. The alexandrite variety of chrysoberyl is one of the rarest and most sought after of all gems.

Occurrence:Tanzania, India, Russia, Madagascar, Sri Lanka, Zimbabwe, Zambia.

Alexandrite is the name given to chrysoberyl which changes color due to the light source under which the stone is being observed. The two colors are usually blue-green in daylight and purplish-red under incandescent light. Stones with a weak change or better are identified as alexandrite while stones with a faint change are considered to be chrysoberyl.

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 spectrum's. 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.

 
 
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