Knischka ruby is a synthetic ruby reportedly grown by a flux fusion method, attributed to Peter Otto Knischka in Austria, though its precise origin and the existence of the named individual remain unverified by major gemmological laboratories. The material is distinguished within the synthetic ruby trade by its strong, even, blood-red colour, its frequent inclusion of natural-looking crystal solids and feathers, and its production in faceted as well as crystal form. Knischka established a small but stable trade for his stones during the 1970s and 1980s as a high-end alternative to flame-fusion synthetic rubies, which had become commercially commonplace by then.

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Method of growth

Knischka used a proprietary flame-fusion (Verneuil) technique, growing the ruby crystals from a molten alumina feed with chromium oxide, without the use of a flux or natural ruby seeds. The process is slow, with growth times of several months per batch, and produces crystals with a structure and inclusion population significantly more like natural ruby than the flame-fusion product of Verneuil. The Knischka stones grew in well-formed hexagonal habit and could be cut to recover round, oval, cushion and step-cut faceted gems with an appearance, on first inspection, very similar to fine Burmese material.

Identification

The GIA, the SSEF and the Gubelin laboratories all identified Knischka ruby in the 1980s as a flux-grown synthetic, distinguishable from natural ruby by characteristic flux-residue inclusions, by angular growth lines, and by the absence of the natural inclusion suite of rutile silk and crystal solids that mark Burmese, Mong Hsu and Mozambique stones. The principal identification features published by SSEF and the GIA Gems & Gemology editorial of the period include curved growth lines, a UV fluorescence response very strong in long wave that exceeds that of most natural rubies, and an absence of natural inclusions. Knischka stones could also be identified by their internal cleanliness in regions away from the flux residues, an unnatural homogeneity for a ruby of any natural origin.

Trade position

Knischka ruby occupies a niche position in the synthetic-ruby trade, distinct from the much larger flame-fusion product. The stones were sold at prices well above flame-fusion synthetics, generally as decorative or experimental gems for the European trade rather than as concealed substitutes. Knischka himself was forthcoming about the synthetic origin of his material and supplied identification details to the major laboratories. Production has substantially ceased since the late 1990s; faceted Knischka stones now appearing in the trade are typically older inventory rather than recent production, and command modest premiums among collectors of synthetic gem history.

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