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What does artificial alexandrite look like?

To understand the fundamental difference between natural alexandrite and its analogues and imitations, let’s try to find out the essence of its unique ability and property to change color. What causes the color change in alexandrites?
Alexandrite is a trichroic gemstone that can absorb and reflect light differently along each of its three optical axes. However, trichroism is not at all responsible for such a remarkable property of alexandrites. The color change is a result of the presence of chromium +3 ions and the way they are embedded in the crystal lattice. In rubies, the absorption spectrum line for chromium +3 ions corresponds to a wavelength of approximately 550 nanometers, and in emeralds, accordingly, approximately 600 nm. In alexandrite, the wavelength of the absorption line of chromium +3 ion corresponds to approximately 580 nm. That is, almost halfway between a red ruby ​​and a green emerald! Daylight is polarized with a predominance of blue and green colors, respectively, and alexandrite appears green. In artificial light, the predominant polarization is red, and alexandrite appears red under a lamp or candle. If a mixed light source is used to examine alexandrite, for example, illuminating it with a flashlight in daylight, then the stone will show a mixed color. It is very important to know that modern daylight fluorescent lamps provide mixed lighting with some predominance of the daylight spectrum. In such lighting, manifestations of the alexandrite effect are practically not observed. Artificial lighting Mixed lighting Natural daylight color And now a little about terminology. Imitations natural precious stones are products that are only similar in appearance to them – these can be either artificially created materials or cheap natural or synthetic mineral substitutes. Roughly speaking, imitations are fakes of natural stone. Synthetic analogues – artificially grown minerals that correspond to natural stones in chemical composition and most physical and optical characteristics (and sometimes slightly superior to them in this regard). A synthetic analogue is the same mineral, only not created by nature, but grown “in vitro”. Stand somewhat apart “ennobled” stones – natural formations whose “consumer” properties, in particular color, are enhanced or changed by heating, x-ray irradiation, impregnation with dyes, polymers, etc. “Refined” alexandrites have not yet been seen on the market, but often the pale greenish-yellow cymophane is given a rich honey-brown color using intense gamma irradiation. At the end of the 2th – beginning of the 3th centuries, synthetic corundum (AlXNUMXOXNUMX) was artificially produced on an industrial scale, which quite soon began to be used as a substitute analogue of natural ruby ​​and sapphire in inexpensive jewelry and costume jewelry. The admixture of vanadium and titanium in the production of synthetic corundum gave it an alexandrite effect with an intense reversal from a faint bluish-greenish-gray to a rich red-violet, purple (amethyst) color. This is the most common imitation natural alexandrite. Also in the mid-20th century, some varieties of synthetic spinels were used to imitate alexandrite. Technical imitations Alexandrite is usually presented as a doublet, in which the upper part is composed of a dark red stone – usually almandine garnet or pyrope, and the lower part is composed of cheap green stone or glass. First information about laboratory production synthetic analogue chrysoberyl-Alexandrite dates back to the second half of the 1851th century. P.A. Chirvinsky, in his classic work “Artificial Synthesis of Minerals in the 5th Century,” mentions that Jacques Joseph Ebelmin obtained synthetic chrysoberyl in crystals up to 6–XNUMX mm in size in XNUMX. He also obtained artificial phenakite and a compound of beryllium and chromium oxides that has no natural analogues, which very accurately imitates the optical properties of alexandrite and differs from it only in its chemical composition (chromium oxide instead of aluminum oxide). Chrysoberyl was also artificially synthesized by A. Saint-Clair Deville and Caron in 1858 and P. Otdeil and A. Pery in 1888.
A. Lacroix in 1887, in a “Note on methods for obtaining corundum,” writes that he was able to artificially obtain needle-like crystals of the rhombic system, consisting of beryllium and aluminum oxides (chrysoberyl composition). He further mentions that the addition of chromium chloride to the original growing mixture gave the crystals a green color that “disappeared when examined in artificial light, similar to what is observed in a variety of chrysoberyl – alexandrite».
In the middle of the 3th century, the growth of synthetic alexandrite using the “solution-in-melt” method was carried out by E. Farrell and J. Fang at the Massachusetts Institute of Technology, but the size of poorly formed alexandrite crystals reached no more than 4,4 mm. A little later, W. Bonner and L. Van Eutert from the Bell laboratory used the same method to grow alexandrite crystals up to XNUMX cm in size.
An industrial method for the synthesis of chrysoberyl-Alexandrite using the flux method from a solution in a lithium molybdate melt was developed in 1964 in the USA. The production of synthetic raw materials using this method, patented by K. Kline and D. Patterson, on an industrial scale began in 1972 by the Creative Crystals laboratory in Saint-Ramond, Danville, California. The essence of the patented method is to slowly cool a solution of BeO and Al2O3 in Li2O+MoO3 from a temperature of 1200°C at a rate of 1°C/hour. Beryllium oxide and alumina make up about 4% of the melt mass, and iron oxide (Fe2O3) and chromium (Cr2O3) – about 1%. In fact, the iron content varies from 0,7 to 2,8%, and chromium from 0,001 to 0,5%. Natural or synthetic chrysoberyls are used as seed crystals, which are placed in a platinum frame and dipped into the melt before cooling begins. Growth continues for 7 to 9 weeks, then the crystals are cut to separate the alexandrite from the seed. This method produces the most beautiful synthetic alexandrites suitable for jewelry purposes, and it remains one of the leading methods to this day. The largest crystals are obtained by the Czochralski method – by pulling the crystal from the melt. The production of synthetic alexandrite by this method has been carried out since 1970 by the Japanese company Kyosera (Kyoto Ceramics Co.). Its products were marketed under the names “inamori” and “cresent-vert” Synthetic alexandrite-inamori has a cat’s eye effect, its reverse from greenish-yellow in daylight to red-violet in artificial light. Since 1980, the Japanese company Seiko has been synthesizing alexandrite using the zone melting method. Synthetic alexandrite Czochralski Synthetic alexandrite grown in flux Due to its special optical and physical properties, alexandrite is valued not only as a gemstone, but also as a technical material, especially in quantum electronics, in particular in the production of lasers with smooth changes in wavelengths in the range of 700–815 nm. They can be used for spectroscopy, annealing of semiconductors, in atmospheric lidars, rangefinders, in the processing of solid materials, and in medicine. Tunable alexandrite lasers have now found particularly widespread use in medical cosmetology and eye microsurgery. Due to its high purity and relative cheapness compared to natural stone, synthetic alexandrite is indispensable as a high-tech material. In 2003, the University of Rochester in New York developed a revolutionary method for tunably changing the speed of light using an alexandrite crystal and two solid-state lasers, one of which is tunable (Alexandrite). With the “standard” speed of light in a vacuum being 300 thousand kilometers per second, scientists were able to reduce it to 91 meters per second, and also significantly accelerate light. This technique (authors Matthew Bigelow and others), unlike the previous ones, is operational at room temperatures and does not require expensive cooling equipment; it can mark the beginning of a new era in the means of transmitting and exchanging data in computer technology and unique research in the field of quantum and relativistic mechanics. In Russia, since 1980, Laboratory No. 6 of the Design and Technology Institute of Single Crystals (Novosibirsk) has established the production of high-quality synthetic alexandrite using flux methods (G. Bukin’s method), Czochralski, zone melting and hydrothermal synthesis. The synthetic alexandrite grown here is suitable for both technical and jewelry purposes, and the weight of individual crystals grown by the Czochralski method reaches half a kilogram. The cost of faceted inserts from a synthetic analogue of alexandrite ranges from 50 to 200 dollars per carat (1/5 gram), i.e. After artificial jewelry diamonds, these are the most expensive synthetic gemstones.
Synthetic analogues correspond to natural alesandrite in chemical formula and most physical and optical properties. However, natural alexandrite remains unsurpassed in both beauty and value. No matter how we feel about synthetic gems, they are a reality of our time and sometimes make childhood dreams come true for less money to acquire the likeness of your favorite stone. In conclusion, I would really like to give a lengthy quotation from the “Essays” of D.N. Mamin-Sibiryak (1884), who in one paragraph managed to express his attitude towards natural and synthetic stone and even make a completely fulfilled forecast for the future: “In general, we can only say that the time of stones seemed to have passed, and our grandmothers understood the silent language of gems better than their granddaughters. In the future, when cheap fake stones begin to play, people will probably be completely freed from this passion for decorating themselves with gems, just as they do not decorate themselves now with simple glass, which is still in fashion among savages who have not become acquainted with glass factories. We think that a native gem will always remain valuable, only as an item for mineralogical collections and for those incorrigible lovers who would not exchange it for any trick of the latest technology. For such lovers, a gemstone is not a dead mineral, but something alive, endowed with living properties. In fact, it was not for nothing that this forest of superstition, poetry and religious analogies grew around the gem. The stone has its own life, dark and unexplored, manifested in the form of crystallization, in the accompaniment of known rocks, in antipathy towards others, in relations to light, electricity and chemical reagents. It is this crystalline form that stands on the border separating organic nature from dead matter, and the human eye inquisitively seeks here an answer to its internal properties, needs and dark organic movements. The dead earth looks at a person with these colored eyes, talking about the secrets of life hidden in it. This is the “last smile” of disorganized nature numb in the dead cold.” Alexandrite is a stone that resembles an emerald during the day and a ruby ​​at night. It is famous for its color changing effect. In sunlight, the stone takes on a greenish tint; in artificial light, it becomes reddish or purple. Alexandrites from a deposit in the Urals are considered standard: on the world market they are called “Russian”. Alexandrites are mined only in a few places on the planet, so jewelry-quality samples are highly valued. Fortunately, man has long learned to create precious stones no worse than natural ones. Synthetic alexandrites resemble the best Ural specimens: clean, bright, with a strongly pronounced reverse.

History of synthesis

Alexandrite is a type of the mineral chrysoberyl. The first attempts at its synthesis were made in the second half of the 5th century: the French chemist Jacques Joseph Ebelmain was able to obtain chrysoberyl crystals measuring 6-XNUMX mm in size. He was the first to synthesize a mineral compound that has no analogues in nature, but with the optical properties of alexandrite. To obtain synthetic chrysoberyl, Ebelman and his students calcined beryllium and aluminum oxides in the presence of mineralizers. The crystals turned out to be small, with numerous flux inclusions. They were not suitable for jewelry purposes. In the mid-twentieth century, attempts were made in the USA to obtain synthetic alexandrite from a solution in a melt. At first the crystals were small, but by 1964 it was possible to develop a synthesis method suitable for use on an industrial scale. In 1972, the Americans K. Kline and D. Patterson filed a patent for the method of flux from a solution in a lithium molybdate melt. In 1970, the Japanese company Kyosera began producing synthetic alexandrites obtained by the Czochralski method – by drawing a crystal from a melt. They were called “kresent-vert” and “inamori”. Alexandrites “inamori” had a cat’s eye effect – a running glare on the surface. Currently, alexandrites are synthesized using the zone melting method and the Czochralski method. The resulting crystals are used for both jewelry and technical purposes. Synthetic alexandrites of the best quality are obtained by the Czochralski method.

How to grow alexandrites

To obtain synthetic alexandrites, low-quality natural stones are used. The crushed raw materials are heated and melted. To obtain the desired color, vanadium and chromium oxides are introduced into the melt. The synthesis process is carried out in a special container, or crucible. It must withstand high temperatures, so it is made of refractory metals – for example, iridium. This platinum group metal costs 3 times more than gold, which affects the final cost of synthetic alexandrites: they cannot be called cheap, although they are cheaper than their natural counterpart. During the synthesis process, a seed of sapphire or other stone, mounted on a vertical rod, is placed in the melt. Then it gradually rises, rotating and turning, and “pulls” part of the melt behind it. This hardens to form an alexandrite crystal. The cycle is repeated several times until the melt is exhausted.

Chemical properties

Alexandrite is not only a beautiful stone, but an extremely hard one. On the Mohs scale its hardness is 8.5. This means that the mineral is practically not scratched, and jewelry with it can be worn every day. Synthetic alexandrite has the same hardness as natural alexandrite. Its edges do not wear off during wear and remain smooth and neat. And if jewelry with an expensive natural stone may be unsafe to wear every day, then with a synthetic one it is completely safe.

Comparison of characteristics

When evaluating alexandrite, the color, severity of the reverse, clarity, weight and quality of cut are taken into account. For example, low quality natural stones weighing 3-5 carats cost on the world market from $600 per carat. Alexandrites of the same mass, but of excellent quality – from 17 thousand dollars per carat and above. Large stones are valued even higher: the record is about 70 thousand dollars per carat. Synthetic alexandrites are graded for the same characteristics. They cost much less.

Color

In daylight, alexandrite displays a palette of shades ranging from yellow-green to green and bluish-green. Yellowness makes the stone cheaper. The most valuable are alexandrites, which turn green or bluish-green when exposed to natural light. Under artificial light, the stone takes on a reddish color, sometimes with an orange or purple tint. Red color without additional shades is the most valuable. Synthetic alexandrites have purer colors than natural ones. This is due to the absence of iron impurities in the crystal. In daylight they look green without a yellow tint, in evening light they look crimson. Some stones also exhibit a cat’s eye effect. The cost of natural alexandrite also depends on how pronounced the color change is. It can be strong, medium or weak. Synthetic alexandrites are characterized by a strong reverse: in this they are comparable to the best Ural specimens.

Cleanliness

Most natural alexandrites have minor inclusions – single points, gas-liquid veils, microcracks. They are not always visible to the naked eye, but they reduce the cost of the stone. Synthetic alexandrites obtained by the Czochralski method are very pure. They contain almost no inclusions.

Weight

Natural alexandrites are usually small: stones weighing less than 2 carats are common on the world market. Larger specimens are found, but much less frequently. The Czochralski method makes it possible to grow really large crystals – both for jewelry and for technical purposes. The weight of individual specimens reaches 0.5 kg or more.

Cut shape

For alexandrite, the shape and quality of the cut are important: it should emphasize the color and optical effects inherent in the stone. The most popular options: Alexandrites with a cat’s eye effect, as well as opaque stones, are treated with cabochon. Natural stones often do not have an ideal cut: in order not to cut off too much material, the cutter deliberately deviates from the standard – for example, skips some of the faces, or changes their angles. This negatively affects the visual perception of alexandrite and reduces its value. Synthetic stones have a standard cut. The master does not need to save on material, and therefore the proportions are precisely adjusted.

How to distinguish synthetic alexandrite from natural one

The differences between synthetic and natural alexandrite are noticeable under a microscope. Gemologists focus on the number and nature of inclusions. The easiest to recognize are stones synthesized by the Czochralski method – they are very pure. In addition, synthetic alexandrites exhibit stronger reversal and fluoresce red in ultraviolet. Cat’s eye stones produce a faint fluorescence – orange-red on the inside, and pale yellow on the surface. They are also distinguished using a microscope. Synthetic alexandrites are dominated by fine silty particles, while natural ones are dominated by tubular and needle-shaped inclusions.

How to distinguish synthetic alexandrite from imitation

  • synthetic corundums;
  • synthetic spinel;
  • doublets;
  • cubic zirkonia;
  • nanotals.

Synthetic corundum is the most successful imitation. A crystal colored with vanadium turns pale green in daylight, and pink in artificial light. It can be recognized by its color palette, inclusions in the form of bubbles, as well as a narrow, clear line in the blue region of the spectrum.

Synthetic spinel was used to imitate alexandrites in the USSR. It is distinguished using a refractometer. Spinel is characterized by the absence of birefringence, a significantly lower refractive index, and a less diverse and plausible range.

Doublets are made from garnet and glass. The garnet top imitates the redness of alexandrite, while the glass top imitates the green. They differ from the original in hardness and other physical properties, and are also inferior in gloss.

Cubic zirconia has almost twice the density of alexandrite. In synthetic crystals, inclusions and growth lines will be noticeable under a microscope, but not in artificial cubic zirconia and nanotals.

Is synthetic alexandrite better or worse than the original?

The main advantage of synthetic alexandrite over natural alexandrite is price. Although due to the high cost of production it cannot be called completely cheap, it is still much cheaper than the original. Visually, the stone is indistinguishable: to see the differences, you need a microscope.

Alexandrite synthesized by the Czochralski method has pure and bright colors. It exhibits a strong reversal: it turns from red to green when the lighting changes, and vice versa. Natural stones with such a pronounced color change effect are rare.

Synthetic alexandrite is practical. If you love this amazing stone, you can wear jewelry with it every day without worrying too much about possible loss. Jewelry with natural alexandrites, which cost fabulous amounts of money, will most likely remain in the box most of the time.

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