Myths and legends

What is artificial alexandrite?

Man-made, synthetic or GNC alexandrite, also known as chromium-bearing chrysoberyl, is an alexandrite obtained through a process of horizontal directional crystallization. The history of the synthesis and cultivation of artificial chrysoberyl (Alexandrite) has several important stages.

History

In the second half of the 19th century, the first experiments on the synthesis of BeAl were carried out2O4, which involved melting or calcining beryllium and aluminum oxides in the presence of various mineralizers. These experiments led to the formation of tiny crystals of chrysoberyl with flux inclusions. In 1944, an attempt was made to grow chrysoberyl crystals from a solution in a melt. For this purpose, mixtures of lead oxide and fluoride, as well as molybdenum oxide with lithium molybdate, were used as solvents. By heating a mixture of flux with oxides of beryllium, aluminum and chromium in a platinum cup to 1375 °C and subsequent cooling, crystals 1–2 mm in size were obtained containing inclusions of the container material and flux. Subsequently, various attempts were made to obtain chrysoberyl crystals, including the hydrothermal method, the Verneuil method, the floating zone method, and the gas transport method. However, the quality and size of the resulting crystals did not meet the requirements for practical use. In 1974, the Institute of Geology and Geophysics of the SB RAS of the USSR began studying the conditions for crystallization of chrysoberyl from a melt using the Czochralski method. In a relatively short period of time, technological methods for producing chrysoberyl and alexandrite crystals were developed. The crystals obtained as a result of these studies measured up to 120 mm in length and up to 40 mm in diameter and had excellent optical quality. This made it possible to create the first solid-state tunable laser.

Horizontal directional crystallization method

Horizontal Directional Crystallization (HDC) is a crystal growth method that provides high quality products, but requires complex technical solutions. The Czochralski method (CZ), which ensures high quality crystals, requires constant monitoring of the geometry of the growing crystal and its mass. Special weighing systems automatically measure the increase in crystal mass or the loss of melt from the crucible. In 1986, in the laboratory of melt crystallization of the Institute of Geology and Geophysics SB RAS, studies began on the conditions for growing single crystals of chrysoberyl and its chromium variety – alexandrite using the method of horizontal directional crystallization (HDC). This method, also known as the Bagdasarov method, is a high-temperature variation of the Bridgman-Stockbarger method. The process involves bending a molybdenum sheet 0,2–0,5 mm thick into a boat shape, into which a charge with the chemical composition of the crystal being grown and a seed are placed. The boat is placed on the molybdenum platform of the growth chamber and subjected to heating. After pumping out the air and establishing the required temperature, the charge is melted, and the boat moves out of the heater zone at a given speed. The melt begins to crystallize on the seed, maintaining its crystallographic orientation—hence the name “directed” crystallization. This method produces high quality crystals and is widely used in research and industry. The process of growing crystals of chrysoberyl and its doped varieties using horizontal directional crystallization (HDC) has been subjected to research that has led to the development of an improved approach. In this method, the initial melting of the charge can be either complete or partial, which is also known as the “floating zone” method – especially in the case of partial melting in the middle of the heating zone. It has been established that before melting at a temperature of 1867 °C, the structure of chrysoberyl undergoes a polymorphic transition of the first type (t=1853 °C). The new phase, formed due to a decrease in the coordination environment of aluminum, has a significantly larger specific volume and loses the ability to replace aluminum with d-elements of the 4th period (chromium, vanadium, titanium and iron). The high-temperature modification of chrysoberyl becomes stable only near the melting point and, upon cooling, transforms into a low-temperature form (chrysoberyl). This leads to severe cracking and the material taking on a chalky appearance. Since the melting of chrysoberyl always occurs after a polymorphic transition, a melt is formed that retains the “memory” of the structural motif of the melting substance. This metastable process makes it difficult to use the classical GNC scheme for stabilizing the low-temperature modification of chrysoberyl. To solve this problem, a method of melt stabilization and seeding was developed, which makes it possible to introduce a seed after complete melting of the charge and normalize the melt. This technique has been developed to the level of industrial technology, providing an effective process for growing crystals of chrysoberyl and its alloyed varieties. Comparison of crystals obtained by horizontal directional crystallization (HDC) and the Czochralski method is an important aspect of studying the technology of growing alexandrite. Alexandrite has attracted attention in the field of laser technology, and the optical quality of the grown crystals has been analyzed.

Differences between GNK crystals and Czochralski crystals

It turned out that the radiation resistance of GNK-Alexandrite is significantly higher compared to its Czochral analogue. This is due to the presence of submicron inclusions of container materials in the bulk of the crystals. In the GNC process, the large free surface of the melt and its active convective mixing in the near-surface zone contribute to the self-purification of the material due to the removal of microparticles to the surface. It was found that in crystals grown by the GNC method, the number of microinclusions of the container material is much smaller and decreases as the growth process progresses. In addition, alexandrite also finds use in the jewelry industry, highlighting its importance in various fields of technology and research. The weak alexandrite effect in GNK crystals may be due to the difficulty of orientation during the cutting process due to the strong pleochroism of alexandrite. Czochralski crystals have a natural facet, which makes them easier to navigate and usually allows for better highlighting of the alexandrite effect. Comparing two stones cut in the same orientation and containing the same alloying impurities does require specialized gemological analysis. This method allows you to accurately determine the method of obtaining the stone. It is interesting to note that crystals of jewelry GNK-Alexandrite reach impressive sizes – about 150 mm in length and weigh about 0,5 kg. This is an impressive achievement in comparison with Czochralski crystals, highlighting the effectiveness and promise of the GNC method in the production of large, high-quality alexandrites. The difference in the behavior of chromium in the GNK and Czochralski processes is of great importance for the production of alexandrites. During the Czochralski method, the crystal actively “sucks” chromium from the melt, which leads to a decrease in the concentration of chromium in the crystal and loss of color. On the contrary, the GNC method partially displaces the impurity, which reduces the uneven color and concentration banding of chromium in alexandrite crystals. This is important to achieve high quality and color purity in cut stones. Regarding the incorrect name “hydrothermal” for alexandrite GNC crystals, this is indeed a mistake. Hydrothermal crystals are grown from solutions at elevated temperature and pressure, while GNC is a melt technology method. Therefore, it is important to differentiate between these methods to avoid misunderstandings and misinformation. Alexandrite is a rare gemstone, a variety of chrysoberyl. Hardness is rated as 8,5 on the Mohs scale (for example, jade has this indicator 6-6,5), which allows us to call alexandrite hard. The stone was discovered in 1834, on the same day as Alexander II came of age. The stone was named in honor of the Emperor. Since 1842, the stone has also been known as jewelry. It was quickly appreciated by the ladies of the court – it was played by the patriotic Russian name and the colors of Russian luxury – green and red, which the stone showed. In the 19th century, the only thing known alexandrite deposit – in the Urals, the stone was considered an original Russian treasure. Only after several decades Other deposits have been discovered around the world. But the Ural alexandrite remained the standard thanks to the iridescence of color and the most intense ability Nowadays, this stone is mined in Sri Lanka, India and Brazil. Natural Ural alexandrites are bluish-green, stones from Sri Lanka and other deposits are more green in color. Brazilian alexandrites, which are found in modern jewelry, have a pale green, olive tint, and faint tints of color. The main feature of alexandrites is their color changes. It is described as an emerald by day and a ruby ​​by night. Natural stones, depending on the lighting, change from cherry red to green. Color and intensity may vary and go into very dark shades. This property is due to the special structure of the lattice. The chromium ions are arranged so that the absorption wavelength is between the wavelength of ruby ​​and emerald. In daylight, polarization occurs with blue and green, and in artificial light – with red. Therefore, alexandrite is green during the day and red in the evening. Usually the stone has low transparency and numerous inclusions, which also affects the color change: green and red have a strong brown tint. Also, the change is not very contrasting – dark green changes to black and red, etc. Stones with clear color changes and high transparency do not go on general sale, but go straight to world-famous jewelry companies or private collections.

Synthetic alexandrite

In the 1950s-1970s, jewelry with synthetically grown corundum, which changes color, first appeared in the Soviet Union. Artificial stone quickly captivated buyers with its intense colors and tints of colors – from blue to purple and pink. Because of this color change, the stone was immediately called alexandrite. Although these are completely different groups of minerals with different physical and chemical characteristics. Synthetic alexandrite most often turns out to be synthetic corundum, which was colored with vanadium during the growing process. Green-colored artificial spinel is also occasionally found as alexandrite. The technology for producing synthetic alexandrite – growing a crystal under artificial conditions – is extremely complex and expensive. Therefore, even artificially grown alexandrite is very rare. The most common type of cut for alexandrite is step or diamond teardrop. It is extremely rare to find products with cabochon-cut alexandrite inserts. Traditionally, the stone is set in precious metals – gold, silver. There are also jewelry with alexandrites in combination with other precious stones. Unfortunately, the vast majority of products with alexandrites were melted down after the collapse of the USSR. This makes jewelry with this stone rare and expensive.

Soviet prejudices

There are many prejudices associated with alexandrite. The most popular of them: alexandrite stone cannot be worn alone, it certainly needs a pair. This Soviet-era prejudice can still be found today. However, the whole meaning of this Russian tradition was lost during Soviet times. Initially, a woman was prescribed to wear alexandrite in a pair only after the death of a loved one (family member), and only in the event of such an event. The prejudices associated with alexandrite originate from the tradition of wearing this stone in the Russian Empire. They were determined by historical events. Over time, the story changed and acquired a different meaning. In the Russian Empire, alexandrite became an analogue of the Victorian mourning stone. This happened in a historical context after the assassination of Tsar Alexander II in March 1881. It was then that the stone experienced a new peak of popularity, but in a mournful context. At this time, it became fashionable to wear alexandrite in a pair – but paired with two diamonds, which was considered a symbol of Alexander II himself and his two main reforms – the abolition of Serfdom and the establishment of new legal proceedings. Later, alexandrite was finally established in the Russian Empire as a mourning stone. However, in Europe and Asia, the attitude towards this stone was the opposite – alexandrite was considered a stone of fidelity and longevity. Knowing the origins of this superstition, you should not be afraid to wear a single piece of jewelry with alexandrite. This is an amazingly beautiful stone and fully deserves to play the “main role” and “solo part” in your image.

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