Often described by gem aficionados as “emerald by day, ruby by night,” alexandrite is the very rare colour-change variety of the mineral chrysoberyl. It can be green in daylight or fluorescent light, and changes to brownish or purplish red in incandescent light from a lamp or candle flame. The Gemological Institute of America explains the reasons why.
Alexandrite’s chameleon-like ability to change colour is the result of the complex way the mineral absorbs light. Its 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 colours when viewed from different directions. Typically, its three pleochroic colours are green, orange and purple-red.
Fine alexandrite is exceptionally rare and valuable. Because of its scarcity, especially in larger sizes, it is a relatively expensive member of the chrysoberyl family. It shares its status as a June birthstone with cultured pearl and moonstone.
History and Lore
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 colour change. The gem was named after the young Alexander II, heir apparent to the throne, and caught the country’s attention because its red and green colours mirrored the Imperial Russian flag.
The Ural Mountain deposits didn’t last forever, and production from Russian mines is very limited today. The intense, fine-coloured gems they produced in quantity less than 200 years ago are much harder to come by, but you may still find estate jewellery set with some of the famed Ural Mountain alexandrites, which remain the quality standard for this phenomenal gemstone.
Most alexandrite today comes from Sri Lanka, East Africa and Brazil. The newer deposits contain some fine-quality stones, but many display less-precise colour change and muddier hues than the 19th- century Russian alexandrites. Alexandrite supply is low and fine-colour material is extremely rare.
Coloured stone professionals assess fine alexandrites by the extent of the colour change they display and by the quality of the red and green hues they show under different lighting conditions.
Colour change is the most important quality factor for alexandrite. The most-prized alexandrites show a strong colour change from bluish green in daylight and red to purplish red in incandescent light, with moderately strong to strong colour saturation. Stones that are too light do not reach the quality of colour intensity seen in fine-quality gems; stones that are too dark lack brightness and appear almost black.
Sri Lankan alexandrites are generally larger than their Russian counterparts, but their colours 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 colours that rival the Russian material, but production from Brazil has decreased.
Most fine alexandrites tend to contain few inclusions and there is a dramatic rise in value for clean material with good colour change and strong hues. If the gem has certain types of long, thin inclusions oriented parallel to each other, they can create an additional phenomenon called chatoyancy, or the cat’s-eye effect, which increases the alexandrite’s value.
Cut is one of the most important factors in appearance. Alexandrites are most often fashioned as mixed cuts, which have brilliant-cut (kite-shaped and triangular facets) crowns and step-cut (concentric rows of parallel facets) pavilions.
Alexandrite’s pleochroism makes it a challenge for cutters, who orient the gem to show the strongest colour change through the crown. It’s crucial to position the rough so the fashioned stone shows both purplish red and green pleochroic colours face-up.
Most fashioned alexandrites are small, weighing less than one carat. Larger sizes and better qualities rise dramatically in price: Fine-quality stones in sizes above 5 carats are very expensive.
Care and Cleaning Guide
Alexandrite is relatively hard at 8.5 on the Mohs scale of mineral hardness. It has excellent toughness and no cleavage (a tendency to break when struck). Alexandrite is stable under normal wearing conditions – it’s resistant to the effects of heat, light and common chemicals – which makes it a good choice for rings and other mountings subject to daily wear. Warm, soapy water is always safe for cleaning.
Alexandrites are usually not treated, although they might have fractures. A fracture-filled gemstone should only be cleaned with warm, soapy water.