Alexandrite Buying Guide
Alexandrite is the variety of chrysoberyl that displays a change-of-color from green to red. A distinct color change is the primary qualification for a chrysoberyl to be considered alexandrite. Although alexandrite is strongly trichroic, its color change has nothing to do with pleochroism. Instead, like all other color-change gems, it results in a near-equal transmission of the blue-green and red portions of the spectrum, coupled with strong absorption in the yellow. Thus its color is dependant on the spectral strength of the light source. Incandescent light is strongly tilted to the red end, thus causing alexandrite to appear reddish. Daylight, is more equally balanced. Since our eyes are most sensitive to green light, the balance is tipped to the green side. The strength of the color change is related to the difference in the areas of transmission, relative to the absorption in the yellow. The greater the difference, the stronger the color change.
For alexandrite, the quality of the color change is paramount. While the holy grail is a gem whose color changes like a traffic light from green to red, such a stone has yet to be found. In fine examples, the change is typically one from a slightly bluish green to a purplish red. The quality of color change is often referred to by dealers in a percentage basis, with 100% change being the ideal. Stones that display a change of 30% or less are of marginal interest and are arguably not even alexandrite. Significant brown or gray components in either of the twin colors will lower value dramatically.
In terms of clarity, alexandrite is comparable to ruby, with clean faceted stones in sizes above one carat being rare and extremely rare in sizes above 2–3 carats. Negative crystals and parallel rutile silk are common inclusions.
In the market, alexandrites are found in a variety of shapes and cutting styles. Ovals are cushions are the most common, but rounds are also seen, as are other shapes, such as the emerald cut.
Alexandrite is one of the world‘s most expensive gems, with prices similar to those fetched by fine ruby or emerald. But like all gem materials, low-quality (i.e., non-gem quality) pieces may be available for a few dollars per carat. Such stones are generally not clean enough to facet.
Facet-quality alexandrite rough is extremely rare. Thus even melee (less than 0.5 ct.) can sell for thousands of dollars per carat. Any fine faceted alexandrite above two carats should be considered quite large. Stones of quality above five carats are extremely rare. While Sri Lanka has produced some alexandrites above 10 carats, these generally do not display a good color change, moving from green to brown.
The name “alexandrite” was coined by mineralogist Nordenskjöld, in honor of the former Russian czar, Alexander II, who came of age about the time the gem was discovered (supposedly on 23 April, 1830). An added factor was that the old Russian imperial colors of red and green are also the colors of alexandrite.
The original locality for alexandrite is Russia. Fine stones have also been found in Sri Lanka, Zimbabwe, Burma, Tanzania, Madagascar, India and Brazil. In 1987, an important new deposit at Hematita, Brazil was discovered. This mine produced for just a brief period, but a number of fine stones were found. In the mid-1990’s, Tanzania’s Tunduru region has also produced some outstanding specimens.
While true synthetic alexandrites do exist, the vast majority of such stones are actually synthetic color-change sapphires colored by vanadium. Since synthetic color-change sapphires have been made from about 1909 onwards, it is entirely possible to have a piece that is nearly an antique. Indeed, many a traveler has returned from a third-world trip with what they think is natural alexandrite, only to later discover (or have their heirs discover) that what they have is a cheap synthetic sapphire worth but a few dollars per carat.
Properties of Alexandrite
|Alexandrite (a variety of chrysoberyl)|
|Refractive Index||1.746–1.755 (0.009) Biaxial positive|
|Colors||Daylight: Green to blue-green
Incandescent Light: Purple to purplish red
Alexandrite is colored by the same Cr+3 ion that gives ruby and emerald their rich hues. Rarely, vanadium may also play a part.
|Pleochroism||Strongly trichroic: greenish, reddish and yellowish|
|Phenomena||Change of color, cat’s eye|
|Handling||No special care needed|
|Enhancements||Generally none; occasionally oiling, dying|