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Alexandrite:
First and foremost, one needs to look at the quality and strength of the color change. A strong change where the colors are bright and attractive under any kind of light is most desirable. Many stones are either a beautiful green in daylight or a nice pinkish red under incandescent light but they are less often attractive under both light conditions. However, since alexandrite is so rare, even stones with a weaker or a less attractive change are still valuable. We grade color changes as faint, weak, moderate, or strong.
Clarity, cut, and size are the next most important considerations after color. Large stones are always rare and as a result more valuable. Cutting and proportions are important but a careful balance between weight, shape, color change and, careful positioning of unremovable inclusions is required to produce the most valuable stone. A poorly cut 2ct.+ stone may be worth more than a perfectly cut .50ct. stone so cutting and preforming require careful attention in order to maximize yield and value.
Russian Alexandrites:
Russian alexandrites have the most historical value, fine stones are available from several other deposits. Brazil, India, and Tanzania produce the bulk of today’s production but alexandrites are also found in Sri Lanka and Madagascar. Top stones are extremely rare but all of these deposits occasionally produce exceptional stones. Brazilian stones typically show the best reds under incandescent light but Indian stones are well known for their superior bluish green daylight colors. Tanzania and Madagascar seem to produce the largest stones and some of the stones are exceptional. There is currently hardly any production of Russian stones.
Colors:
Some look at the red incandescent color and price the stones based on the intensity of the red. My own feeling is that both the daylight green and the incandescent colors should be nice and highly saturated if possible. Some stones may only look good in daylight or only under incandescent light and they should be less expensive.
Synthetics:
Synthetic alexandrite like sapphire has been around for almost 100 years. Flux grown or lab created alexandrites have been produced for over 30 years. Synthetic alexandrites are common but can be easily identified by experienced gemologists. Curiously we seem to receive many inquiries about alexandrites purchased in Alexandria, Egypt where there is no known deposit but probably strong demand from the tourists looking for souvenirs. Almost all of these stones are synthetic. Gemologists study inclusions and gemological properties to identify synthetic stones. Alexandrite or Chrysoberyl, What's the difference?
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?
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.
Fluorescence is another property to help us differentiate. Chrysoberyl owns 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 and not chrysoberyl.
Other guides relating to jewelry and gemstone buying which you may find helpful are as follows:
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