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.
What to look for in alexandrite?
First and foremost, we need 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 retention, shape, color change and, careful positioning of unremovable inclusions is required to produce the most valuable stone. A poorly cut 1ct.+ stone may be worth more than a perfectly cut .90ct. stone so cutting and preforming require careful attention in order to maximize yield and value.
Which alexandrites are best?
Although 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.
What colors should we look for?
There is no fixed rule about this. Some dealers look at the red incandescent color and price the stones based on the intensity of the red. In our opinion, both the daylight greens and the incandescent colors should be attractive and hightly saturated if possible. The srength of the color change change is the most important factor affecting the value of alexandrites. The GIA grades the following colors as top in daylight, G 5/2 (medium slightly grayish green) and vslbG 5/2 (medium slightly grayish very slightly bluish green). Under incandescent light, R 5/3 (medium very slightly brownish red) and slpR 5/3 (very slightly brownish slightly purplish red) are considered to be the best colors. Some stones may only look good in daylight or only under incandescent light and they should be less expensive. Finally, it is the buyer that will wear the stone so the colors he or she likes are the most important.
Is my alexandrite real?
Synthetic alexandrite like sapphire has been around for almost 100 years. Flux grown or lab created alexndrites 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.
What causes the color change?
Alexandrite is a trichroic gemstone which may absorb and reflect light differently in each of its three optical directions. However, it is not the trichroism that is responsible for the remarkable change. The color change phenomena is a result of the presence of chromium +3 ions and the way they are absorbed and reflected. In rubies the chromium absorption band is around 550 nanometers and in emeralds, the band is around 600nm. In alexandrite, where the band is at 580nm and right between ruby red and green emerald, the stone is balanced between them. 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, 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? 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 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.
In the case of “borderline chrysoberyls”, (stones showing a visible but weak color change), it may be difficult to find any spectrum or fluorescence. Even the GIA reference guide says that alexandrite can show no fluorescence.
If a color change chrysoberyl shows a chromium spectrum and red fluorescence, it will be named variety “alexandrite”. For a “border line stone” showing a visible color change but an inconclusive spectrum and no fluorescence, the authority issuing the report will then use the master stone to make the decision. If this “border line stone” shows a weaker color change compared to the master stone it will be graded as a chrysoberyl and if it shows a stronger one, it will be graded as an alexandrite.
About our photographs.
Alexandrite is especially difficult to photograph. Not only does the color change as a result of illumination under different kinds of light sources but it also varies as a result of geographical location and time of day. Pictures can also differ because of technological obstacles like screen and graphic resolutions. We try to minimize the variations with good equipment and controlled lighting so that our images are as accurate as possible. The colors in our images are dependent on the lighting environment and we use neutral daylight (~5500K) for daylight shots and studio or photo flood lights (3300 – 3500K) as our incandescent light source. A typical alexandrite shot will maximize the observable color change by obstructing any unintentional ambient light so that the resulting image is a function of a singular unmixed light source. We work with imaging programs like Paint Shop and Adobe for sizing and positioning but we do not alter the colors unless it is too improve the accuracy of the color presentation.
Observing Color Change
To observe the color change, daylight 5500K+ works best to see the daylight color. 5500K is the quality of light you will see on a clear day at noon in the northern hemisphere latitude 41.5° N, (New York City is 40.7127° N). So, if you don’t have the right lights, check the color at noon. Candle light is the ideal incandescent light and it was from candle lights that color change in gemstones was first observed. However, as candle light is impractical, use a warm spotlight or a penlight. Light around 3300K is what most gemologists use but candle light works even better.
Color change is also related to the distance of the light source from the stone as well as the angle of the light to the stone. For observing the red color, the penlight should be 2 to 3 inches from the stone and the strongest affect will occur if the light is directed across the table.