Synthetic alexandrite

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Synthetic alexandrite is a crystal which has been created by the floating zone method. This method is also used to synthetically grow a variety of crystals: YAG (yttrium aluminium garnet) in white, blue, pink and several other shades, ruby, chrysoberyl and, most and foremost, alexandrite. This method is similar to the pulling (Czochralski method), when the crystal is pulled vertically, but, instead, here it is pulled horizontally, using a tungsten dish and slow-melting technique, where the dish is pulled over a period of time under a heating element.

The period of time depends on the size of the dish and the materials needed to be crystallized. In the case of alexandrite, the speed of the process is about 2 mm (0.08 in)/hour. An average crystal being 18 cm (7 in) long would take about 15 days to pull.

This method was developed by an Armenian scientist, Khachatur Saakovich Bagdasarov, of the Institute of Crystallography, in 1964 and was widely used in production of white YAG for spacecraft and submarine illuminators, before finding its way into the jewelry scene.

Flux-grown alexandrite is more difficult to identify because the inclusions of undissolved flux can look like natural inclusions. Alexandrite grown by the flux-melt process will contain particles of flux, resembling liquid feathers with a refractive index and specific gravity that echo that of the natural material. Layers of dust-like particles parallel to the seed plate, and strong banding or growth lines may also be apparent. Some stones contain groups of parallel negative crystals. Flux grown alexandrites are more difficult to spot because the colors are convincing and because they are not clean. These stones are expensive to make and are grown in platinum crucibles. Crystals of platinum may still be evident in the cut stones.

Czochralski or pulled alexandrite is easier to identify because it is so clean. Curved striations visible with magnification are a dead give away. The color change in pulled stones has seen change from blue to red, which resemble alexandrite from Brazil, Madagascar, and India. Seiko synthetic alexandrites have a swirled internal structure characteristic of the floating zone method of synthesis. They have tadpole inclusions (with long tails) and spherical bubbles.

The Inamori synthetic alexandrite had a cat's eye variety, which showed a distinct color change. The eye was broad and of moderate intensity. Specimens were a dark greyish-green with slightly purple overtones under fluorescent lighting. The eye was slightly greenish-bluish-white and the stones were dull and oily. They appeared to be inclusion-free and under a strong incandescent light in the long direction, asterism could be seen with two rays weaker than the eye. This has not been reported in natural alexandrite. Under magnification, parallel striations could be seen along the length of the cabochon and the striations were undulating rather than straight, again not a feature of natural alexandrite.

The name allexite has been used for synthetic alexandrite manufactured by the Diamonair Corporation who maintains that its product is Czochralski-grown.

Most gemstones described as synthetic alexandrite are actually synthetic corundum laced with vanadium to produce the color change. This alexandrite like sapphire material has been around for almost 100 years. The material shows a characteristic purple-mauve color change which, although attractive, differs from alexandrite because there is never any green. The stones will be very clean and may be available in large sizes. Gemological testing will reveal a refractive index of 1.759–1.778 (corundum) instead of 1.741–1.760 (chrysoberyl). Under magnification, gas bubbles and curved stria may be evident. When examined with a spectroscope a strong vanadium absorption line at 475 nm will be apparent.


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