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《Incolor》Reading|The mysterious Alexandrite

2020-11-18 Source:GDGJE

The alexandrite is favored by the gemology circle because of its different colors under different light sources. Alexandrite has a color-changing effect (the phenomenon that the color of a gem mineral changes with the change of the spectral energy distribution of incident light or the wavelength of incident light is called the color-changing effect). It looks like “emerald” under the fluorescent lamp and looks like “ruby” under the incandescent lamp. It is mysterious and charming.

Faceted 0.8-ct alexandrite (3x2.2.5x2.5 cm).
Left: In daylight. Right: Incandescent lamp.

In the case of the name of the chrome variety of chryso-beryl—alexandrite—everything happened a bit differently. The mineral was named in 1834 in honor of Tsarevich Alexander, who at the time was only 16 years old, and he had not distinguished himself in any special way.

01  Russia's main Alexandrite Mine

In the Urals, alexandrite and emerald are found in the same geological formation (micaceous), and often in the same mineral associations. About half of all known emerald deposits known in the "Ural emerald mine" literature have discovered alexandrite mineralization.

The main alexandrite deposits in the emerald zone are:
5.Krasno-armeyskoye(Hitny mine);
7.Section 616;
10.Lublinskoe(Named after Krupskaya);

The waste quarry of the Mariinsky deposit. From 1950 to 1971, open pit mining was carried out. The mining depth reaches 125 meters. Today, the underground mining depth has reached 350 meters.

Ore bodies in deposits of the emerald mine are represent-ed by two main types: rare-metal veins, including pegmatite and quartz-plagioclase veins; and metasomatic complexes. Among the metasomatic complexes, the following stand out: 1) graysens on granites; 2) mica complexes by ultrabasites.

The gemstone is mainly associated with the mica complex. In the emerald mining area, a total of 34 deposits and deposits were found, 20 of which were composed of mica complexes, containing emeralds, alexandrite, beryl, beryl, and beryl, which appeared in the form of ore. Gem minerals and collected specimens were found in 11 deposits. Among them, the Glinskoye deposit has mined alexandrite with promising prospects.

02  The Mineralogical Characteristics of Alexandrite

In emerald mines, alexandrite is most often found in the form of tees, called “pawns” in the Urals, with the correct pseudo-hexagonal symmetry. The main Simplex are pinacoid {100}, bipyramid {111}, prism {120}, as well as {010}, {011}, {121}. The most common tees are of the pinacoid-bipy-ramidal habit, although tees are also quite common. Signifi-cantly less common are pinacoid habitus.

In the ore deposits composed of mica complex, the mineral composition of mica containing alexandrite is different from that of mica containing emerald. The former contains higher magnesium content, but lower silicic acid content.The usual paragenesis of alex-andrite is emerald, phenakite, apatite, fluorite, plagioclase,
topaz, margarite, and molybdenite.

Usually, alexandrite contains a significant amount of phlo-gopite inclusions, which indicates the syngenetic formation of these minerals. Quite often, both alexandrite and chrysoberyl form drusen aggregates with apatite, however joint splices were not observed. In micas, as well as in quartz-plagioclase veins, it forms inclusions in fluorite, that is, earlier. In relation to phenakite, alexandrite is also earlier.

03 The Gemological Characteristics of Alexandrite


Top: Skeletal tee of alexandrite in daylight.
Above: Under incan-descent lamp.
Size is 1.5x1.5x1 cm.

A special property of alexandrite is the “Discoloration effect” relating to its ability to change color, depending on the kind of light source. Such a color change is explained by the presence of Cr +3 in the crystal lattice, replacing Al. The “Discoloration effect” is a consequence of the strictly determined position of the bands in the absorption spectrum .

In Russia, in gemological practice, the concept of “reverse” is used, that is, the sum of the brightness values of green and red, expressed as a percentage. The reverse coefficient can vary from 10% to 90% or more. Divide color changes into three groups:
(1) with a strong effect (reverse 70% to 100%);
(2) with medium effect (40% to 70%);
(3) with a weak effect (10% to 40%).

At the same time, its clarity can also be divided into three groups. The first includes transparent stones with inclusions barely visible to the naked eye. The second are transparent stones with a network of inclusions in separate areas visible to the naked eye. The third group are partially transparent stones, with inclusions in the entire volume, visible to the naked eye.

For the coloring of alexandrite, “color purity” is important, that is, lack of brownish shades. The color characteristics of alexandrite and the features of the reverse manifestation are determined both by the content of the main impurity elements, chromium and iron, and secondarily of copper, vanadium and manganese.

04  The Future of Ural Alexandrite

For a long time, alexandrite has received much attention. Like other gemstones, the price of alexandrite depends on size, color, and clarity (no flaws). What are the prospects for Urals? What are the reserves and potential resources of Ural alexandrite?

From 1831 to 1921, emerald (and alexandrite) research official statistics show that about 5 tons of alexandrite have been mined, including more than 82 kg (410,000 carats) of jewelry-grade gemstones, which is equivalent to 1.6% of production.

In the years 1921 to 1971, we estimate that about 600 kg of crystals (about 10 kg of cut material) were mined. From 1972 to 1992, our estimate is that 240 kg of crystals (4-5 kg of cut material) were mined.

For the entire period 1831 to 1992, 5840 kg of crystalline raw material was extracted, including 97 kg (485,000 carats) of high-grade alexandrite. According to Laskovenkova and Zhernakova, reserves of emerald and alexandrite are 30% to 35% (i.e. one-third). Alexandrite in the 11,680 kg main deposit, including 194 kg (970,000 carats) of polished gemstones. But the most optimistic estimate at present is to mine 40 to 50 kg of crystals and produce 1.5 kg of cut gemstones.

The mining of alexandrites, the output of cut minerals and their reserves are difficult to estimate accurately; on the other hand, the deposits where alexandrites are located are very complicated in terms of geological structure and water content, as well as mining and geological conditions. Increased the cost of exploration, mining and production. Nonetheless, various proposals have been made at present, hoping to change the status quo, such as amending relevant bills, reducing corporate tax burdens, introducing new technologies for mineral deposit development and mineral mining.


The content is organized from 《InColor》2020 spring.

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