Alexandrite, Education

ALEXANDRITE

MINERAL: Chrysoberyl
CHEMISTRY: BeAl2O4
COLOR: Bluish green in daylight, purplish red in incandescent light
REFRACTIVE INDEX: 1.746 to 1.755
BIREFRINGENCE: 0.008 to 0.010
SPECIFIC GRAVITY: 3.73
MOHS HARDNESS: 8.5

Sources

Russia, Tanzania, Madagascar, Sri Lanka, Brazil.

Birthstones & Anniversaries

Alexandrite is a birthstone for June, along with pearl and moonstone. Alexandrite is also the gem for the 55th wedding anniversary.

Description

Green in sunlight. Red in lamplight. Color-changing alexandrite is nature’s magic trick.
Often described by gem aficionados as “emerald by day, ruby by night,” alexandrite is the very rare color-change variety of the mineral chrysoberyl. Originally discovered in Russia’s Ural Mountains in the 1830s, it’s now found in Sri Lanka, East Africa, and Brazil, but fine material is exceptionally rare and valuable.
Alexandrite, with its chameleon-like qualities, is a rare variety of the mineral chrysoberyl. Its color can be a lovely green in daylight or fluorescent light, changing to brownish or purplish red in the incandescent light from a lamp or candle flame. This is a result of the complex way the mineral absorbs light.
Alexandrite’s dramatic color change is sometimes described as “emerald by day, ruby by night.” Other gems also change color in response to a light-source change, but this gem’s transformation is so striking that the phenomenon itself is often called “the alexandrite effect.”
Alexandrite is also a strongly pleochroic gem, which means it can show different colors when viewed from different directions. Typically, its three pleochroic colors are green, orange, and purple-red. However, the striking color change doesn’t arise from the gem’s pleochroism, but rather from the mineral’s unusual light-absorbing properties.
Because of its scarcity, especially in larger sizes, alexandrite is a relatively expensive member of the chrysoberyl family. It shares its status as a June birthstone with cultured pearl and moonstone.

ACCESSORIES

History

Abundant alexandrite deposits were first discovered in 1830 in Russia’s Ural Mountains. Those first alexandrites were of very fine quality and displayed vivid hues and dramatic color change. The gem was named after the young Alexander II, heir apparent to the throne. It caught the country’s attention because its red and green colors mirrored the national military colors of imperial Russia.
The spectacular Ural Mountain deposits didn’t last forever, and now most alexandrite comes from Sri Lanka, East Africa, and Brazil. The newer deposits contain some fine-quality stones, but many display less- precise color change and muddier hues than the nineteenth-century Russian alexandrites. You’ll still find estate jewelry set with some of the famed Ural Mountain alexandrites. They remain the quality standard for this phenomenal gemstone.

ALEXANDRITE QUALITY FACTORS

Fine alexandrite is green to bluish green in daylight and red to purplish red in incandescent light.
Good quality alexandrite has few inclusions. Rarely, needle-like inclusions create a cat’s-eye.
Alexandrite is most often available in mixed cuts. Its rarity means it is often cut to save weight.
Most cut gems weigh less than one carat. Larger, higher-quality gems rise in price dramatically.

Color

Fine alexandrite is green to bluish green in daylight and red to purplish red in incandescent light. Its color saturation is moderately strong to strong. Stones that are too light do not reach the quality of color intensity seen in fine-quality gems. Stones that are too dark lack brightness and appear almost black.
Production from Russian mines is very limited today, which means the intense, fine-colored gems they produced in quantity less than 200 years ago are much harder to come by.
Sri Lankan alexandrites are generally larger than their Russian counterparts, but their colors tend to be less desirable. The greens tend to be yellowish compared to the blue-green of the Russian stones, and the reds of Sri Lankan alexandrite are typically brownish red rather than purplish red.
Alexandrites from Brazil have been found in colors that rival the Russian material, but production from Brazil has decreased.
Currently, alexandrite supply is low, and fine-color material is extremely rare.

Clarity

Alexandrite tends to contain few inclusions. There’s a dramatic rise in value for clean material with good color change and strong colors.
When certain types of long, thin inclusions are oriented parallel to each other, they can create an additional phenomenon called chatoyancy, or the cat’s-eye effect, increasing the alexandrite’s value.

Cut

Alexandrites are most commonly fashioned into what are called mixed cuts, which have brilliant-cut crowns and step-cut pavilions. Brilliant cuts have kite-shaped and triangular facets, while step cuts have concentric rows of parallel facets.
Alexandrite’s pleochroism makes it a challenge for cutters. When fashioning alexandrite, cutters orient the gem to show the strongest color change through the crown. It’s crucial to position the rough so the fashioned stone shows both purplish red and green pleochroic colors face-up.

 

Size and Weight

Most fashioned alexandrites are small, weighing less than one carat. Larger sizes and better qualities rise in price dramatically.

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