Alexandrite is a chrysoberyl mineral type characterized by an interesting property: the presence of a color change effect. It is one of the rarest and most expensive gemstones in the world. Gem- quality natural alexandrites are less common than rubies, sapphires, emeralds and diamonds. The first find was made in Russia in the Urals in 1830. Today, alexandrite from the Urals are one of the most “branded” and expensive. The name of the stone was given in honor of the future Tsar of Russia Alexander II. The most interesting feature of it is that it changes its color from green to reddish tones depending on the type of lighting.
Two types of lamps are used to observe the “alexandrite effect”: with daylight range, in which a stone shows green colors; and incandescent lamp where the gem takes on purple of reddish hues. Alexandrite, which has a strong reverse (color change) in bright saturated colors, is the most appreciated. And colors of reverse should be close to pure green and red, without additional shades. Natural chrysoberyl often contains inclusions, therefore characteristics of clarity are not evaluated very strict, the primary factor is the description of color. The price of the top-quality alexandrite can exceed $ 10,000 per carat. Be careful, that most specimens on the market displays a dark color.
Another chrysoberyl type is the alexandrite cat’s eye, also referred as chatoyant or cymophane. This stones in cabochon form are characterized by the presence of a bright strip of light on the surface. The cat’s eye effect is usual for Tanzanian alexandrite. The most popular imitations are synthetic corundum and synthetic spinel with the effect color change. Such “alexandrites” can be given the following description: a violet hue, perfect clarity, standard factory forms of cutting, and they are not at all like natural gems in their physical properties.
Keep in mind that synthetic alexandrites (grown by various methods) are also produced nowadays with similar properties as in natural specimens, which should not be confused with expensive and rare stones of natural origin. Natural faceted alexandrites (chrysoberyls) of good characteristics are rarely exceed weight of 3 carats. Mining is mainly conducted in Brazil, Sri Lanka, Tanzania and Russia (in the Urals). How this stone looks like you can see in the photo below.
Natural gem-quality alexandrite are mined just in a few places in the world: Tanzania, Brazil (Hematita deposit), India, Sri-Lanka, Zimbabwe. Occasionally, alexandrite comes from the Urals, Russia. Russian alexandrite have not bad green color under daylight, in comparison with stones from other countries. But their average size is often smaller and the clarity is worse than stones from other countries. The color of Indian and Sri Lankan alexandrite specimens usually demonstrates brownish-orange shades, and Brazilian and Tanzanian specimens often have bluish and purplish shades.
The color of alexandrites from different deposits may have different shades.
In May 2014, an alexandrite weighted 21.41 carats from the Urals was sold at Christie’s for 1.5 million US dollars, which is approximately 70 000 dollars per carat.
Alexandrite gave the name to the phenomenon of changing colors in gems from greenish to reddish: “alexandrite effect”.
There are two versions of of the discovery of this variety of chrysoberyl. Due to the first version, alexandrite was discovered in the Urals by the Finnish mineralogist Nils Gustaf Nordenskiöld. According to the second version, alexandrite was found at the emerald deposit on the Takova River near Yekaterinburg in 1833, where the stone was described as an emerald by Ya.V. Kokovin. Then his find was studied in St. Petersburg by L.V. Perovsky, who noted that a hardness of the stone was higher than emerald, as well as the fact that the mineral had an interesting property - reverse. The name of the new stone was given on the occasion of the coming of age of the future Tsar of Russia Alexander II. Alexandrites were very popular in the late 19th century in the highest circles of Russian society.
Particullary all alexandrites on the world market are untreated (natural). In practice, alexandrites aren’t improved by any methods. Heat treatment or irradiation can’t change natural color. Only in rare cases clarity of highly cracked specimens can be improved by oiling. And the channels in the cabochons of alexandrite with the effect of a cat's eye can be cleaned with acid.
On the photo: natural alexandrite 5 ct. It displays its rare color changing effect: two colors under different types of lighting.
On the photo: cabochon with cat’s eye phenomenon 4.22 ct.
On the photo: a fragment of the gemstone report of the laboratory GRS on the large alexandrite. It is shown how a stone looks like under different lighting. But in reality, both colors of a stone are usually not as pure as on a photo from a report.
Photo of alexandrite from Tanzania weighted 5.19 ct