Synthetic alexandrite
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Synthetic alexandrite has been created by the floating zone method. This method is also used to synthetically grow a variety of crystals: YAG (Yttrium Aluminum 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/hour. An average crystal being 18 cm long would take about 15 days to pull.
This method was developed by a Russian-Armenian scientist, Professor Bagdosarov, of the Institute of Crystallography, in the 1970s and was widely used in production of white YAG for spacecraft and submarine illuminators, before finding its way into the jewelry scene. The actual method of Alexandrite’s production by this method was developed by Tairus' Scientist, Vladimir Gurov, whom development process of other gems, such as Chrysoberyl and Ruby by this method are also attributed.
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. Although the stones look nice, the colour change doesn't resemble alexandrite from any deposit. 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 colour 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 Diamonair Corp who maintain that their product is Czochralski-grown.
[edit] References
- Methods of producing synthetic alexandrite. Alexandrite Tsarstone collectors guide (2006-09-07). Retrieved on 2006-11-04.
