The veteran Austrian crystal grower Johann Lechleitner is the genius behind the production of synthetic corundum in a variety of colors by the flux process. The experimentation began in 1983. As with Knischka, Lechleitner uses seeds as a focus for growth. His production can be divided into two types depending on the type of seed used.
- Type 1
Growth on Verneuil seeds. These are grown in a variety of colors (red, pink, orange, blue, yellow, green and colorless) with the color of the seed usually matching that of the overgrowth. Inclusions are characteristic of both the flux method of growth (reminiscent of Kashan and Chatham synthetics) and of the Verneuil seed (gas bubbles, curved striae and color banding). This is a characteristic and easily recognized assemblage of inclusions.
- Type 2
Growth on natural corundum seeds. This type of Lechleitner synthetic requires much closer attention. The synthetic overgrowth will adopt the crystal habit of the seed underneath and reproduce growth structures identical to the natural material (i.e. an overgrowth on a Thai ruby seed will produce a tabular crystal of identical habit to the natural crystal). Repeated twinning found in the seed will continue out into the overgrowth.
Inclusions in the seed portion can easily confuse the eye, especially if the identification is hasty. Features typical of flux-growth corundums should be seen in the overgrowth. The best method of detecting type 2 corundums would seem to be with immersion in high RI liquid, but even with this technique it may be difficult to see the boundary between the seed and the overgrowth (this should be present as a narrow colorless layer at the junction). Unfortunately, the growth process seems to use such high temperatures that the surface of the seed may be actively dissolved by the flux, making the junction between seed and overgrowth harder to detect.
The cutting process may also remove much of the overgrowth, in which case the inclusions within the seed would be more noticeable than the flux inclusions left in the vestiges of the seed. One consequence of using a natural seed will be the difference in fluorescence between the two 'components' of the stone. Under longwave ultraviolet light the natural seed will glow a much weaker red than the overgrowth, which will fluoresce bright red.
The stones are rare. You may find the specimens only at a few gemological schools, gem research laboratories and among private collectors.
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