Flux-grown synthetic sapphire is corundum (aluminium oxide) crystallised from a molten flux solvent at high temperature, typically between 1,200 and 1,400 degrees Celsius. The technique, developed commercially in the 1960s, produces synthetic sapphire and ruby with growth characteristics distinct from those of flame-fusion (Verneuil) and Czochralski material, and produces synthetic stones whose inclusion patterns and absorption features can closely mimic those of fine natural corundum.

Process

The flux process dissolves aluminium oxide and the appropriate colour-causing oxides (chromium oxide for ruby, iron and titanium oxides for blue sapphire) in a molten flux of lead oxide, lead fluoride, boron oxide, or a similar low-melting compound. The solution is held in a platinum or platinum-rhodium crucible, slowly cooled over a period that may extend to several months, and crystals are recovered by dissolving the cooled flux in dilute acid. The slow growth temperature, well below the melting point of corundum (approximately 2,040 degrees Celsius), produces inclusion patterns and crystal habits more like those of natural growth than the rapid solidification of the Verneuil process produces.

Producers of flux-grown synthetic sapphire have historically included Chatham (United States, founded by Carroll Chatham), Kashan Inc. (United States, no longer in production), Knischka (Austria), Ramaura (Israel), and Douros (Greece). Chatham material remains in current production. Each producer's flux composition and growth conditions produce characteristic inclusion patterns that gemologists use to distinguish flux-grown synthetics from natural stones.

Identification

Flux-grown synthetic sapphire and ruby present significant identification challenges because the inclusions can closely resemble those of natural corundum. The most diagnostic feature is the presence of flux residue: undissolved flux trapped during growth produces inclusions that may appear as wisps, veils, fingerprints, or breadcrumb patterns. Under high magnification with darkfield illumination these inclusions show a characteristic angular or droplet form, often colourless to white, with high relief that is distinctive to gemologists trained in their recognition.

Additional diagnostic features include platinum platelets from crucible contamination, visible as opaque hexagonal or triangular flat inclusions with metallic lustre; primary growth zoning that differs from the angular hexagonal zoning of natural corundum; and absorption features observable through gemological spectroscopy. UV-Vis-NIR and FTIR spectroscopy provide additional confirmation in difficult cases. The major laboratories — GIA, Gubelin, SSEF, AGL, and Lotus — routinely identify flux-grown synthetics, and the methodology is documented in the gemological literature.

Disclosure obligations

Flux-grown synthetic sapphire must be disclosed as synthetic at every stage of trade. The CIBJO Coloured Stone Book requires that the term synthetic be applied to all laboratory-grown corundum, with the growth method (flux, hydrothermal, Czochralski, or flame fusion) disclosed where requested. The Federal Trade Commission Jewelry Guides in the United States permit the terms synthetic, lab-grown, lab-created, or grown by [producer name] but require that any of these terms be used with the same conspicuousness as the gem name itself. The use of the unqualified term sapphire for synthetic material is prohibited.

The pricing differential between flux-grown synthetic and fine natural corundum is large. A flux-grown synthetic ruby of one carat may retail at thirty to one hundred dollars per carat; a natural Burmese ruby of comparable size and quality might retail at five thousand to fifty thousand dollars per carat. The pricing therefore creates strong incentive for misrepresentation, and the trade has historically taken disclosure obligations on synthetic corundum seriously.

Position in the market

Flux-grown synthetic sapphire occupies a specialised position in the contemporary market. Verneuil synthetic corundum, much cheaper and produced in much greater volume, dominates the entry-level and industrial market. Hydrothermal synthetic corundum is preferred for emerald-equivalent productions where the visual character is to mimic finer natural material. Flux-grown synthetic corundum is produced in smaller quantities and is generally aimed at the higher-quality jewellery market where the inclusion patterns and the visual character matter for the finished product.

Because of the genuine difficulty of distinguishing flux-grown synthetic corundum from natural material in some cases, the trade has converged on the position that any unmounted high-value sapphire or ruby presented as natural should be accompanied by a report from a recognised laboratory that confirms natural origin. The labs themselves rely on a combination of microscopic examination, spectroscopic measurement, and trace-element chemistry to make the determination, and they remain the authoritative source for the distinction.