The Linde star, more accurately the Linde-style synthetic star sapphire or ruby, refers to a class of synthetic corundum produced by the Linde Air Products Company, a division of Union Carbide, and to the technical innovations that allowed flame-fusion synthetic corundum to display asterism. The original Linde commercial production ran from approximately 1947 into the 1970s and the resulting cabochons populate a generation of mid-century estate jewellery in remarkable numbers.

Production

Linde made synthetic corundum by the Verneuil flame-fusion process, the same technique that had been used to make non-asteriated synthetic ruby and sapphire since the early twentieth century. The technical advance was to introduce a small percentage of titanium oxide into the feedstock and then to heat-treat the resulting boule under controlled conditions. The titanium initially dissolved into the corundum lattice during the high-temperature growth and subsequently exsolved as oriented rutile needles during the cooling and post-growth heat treatment. The same mechanism produces asterism in natural star corundum, but the Linde process produced denser, more uniformly distributed needles than typically occur in nature.

Visual character

A Linde-style synthetic star displays a sharp, well-centred six-rayed star against a strongly saturated body colour, most commonly the deep blue of the Linde Star sapphire or the medium red of the synthetic ruby version. Compared with most natural star sapphires the synthetic star is typically more vivid, more sharply defined, and travels across the dome of the cabochon in a more linear fashion. Body colour is generally more uniform and more saturated than a natural stone of comparable grade, although fine natural Burmese and Sri Lankan stars can equal the synthetic in star definition.

Identification

The diagnostic marks of Linde-style synthetic stars are well documented. Curved striae following the boule's growth orientation are visible under magnification with proper illumination and immersion; these are absent in natural sapphire, which shows straight, hexagonal growth zoning. Gas bubbles, sometimes elongated, are diagnostic. The base of the cabochon often shows a flat, slightly concave or polished surface rather than the rough or untreated base of a natural cabochon. Some Linde stars carry a dot or other mark on the base. UV reaction can also be diagnostic, with synthetic ruby showing strong red fluorescence under longwave UV.

Trade significance

The Linde star occupies a particular place in the trade. As a synthetic, it is properly disclosed as such, but it is not a fraud product in the historical sense; the goods were marketed honestly during the production period and were sold openly through established retail and military channels (Linde stars were a popular gift among American servicemen in the 1950s and 1960s). Estate-jewellery valuation today reads Linde stars on their own terms: a well-made setting with an undamaged Linde-quality star and acceptable mounting metal commands a modest but real market, well below comparable natural goods but well above the price of a generic synthetic. The collectability of period American jewellery, particularly servicemen's gift jewellery, supports a stable secondary market.