Linde star sapphire is the trade name for synthetic asteriated corundum produced from 1947 onwards by the Linde Air Products Division of Union Carbide. Linde star sapphires were the first commercially viable synthetic stones to display asterism, the six-rayed silvery star phenomenon characteristic of certain natural sapphires and rubies. The product line included blue, red (synthetic star ruby, marketed as Linde star ruby), black, white, and other colours, and dominated the synthetic-asterism market through the 1970s. Linde production ceased in 1974 when the line was sold to Nakazumi Earth Crystals, but the term remains in active use across the gemmological literature.

Production method

Linde star sapphires are produced by the Verneuil flame-fusion process modified to incorporate excess titanium oxide in the alumina feed. The crystal boules are grown by dropping aluminium oxide powder mixed with colouring agents (chromium for red, iron and titanium for blue, etc.) and additional titanium dioxide through an oxyhydrogen flame, where the powder fuses and crystallises onto a slowly descending pedestal. The titanium oxide remains in solid solution at the high growth temperatures.

The boule is then subjected to a controlled annealing schedule, typically several hundred hours at temperatures around 1,100 to 1,300 degrees Celsius. During this annealing, the titanium oxide exsolves from solid solution and precipitates as oriented rutile (titanium dioxide) needles along the three crystallographic directions perpendicular to the c-axis. When the boule is cut and polished into cabochons with the c-axis perpendicular to the dome, the exsolved rutile produces the six-rayed star characteristic of asteriated corundum.

Identification

Linde star sapphires are distinguished from natural star sapphires by several features. Most diagnostic is the back of the cabochon, which in Linde production is generally flat or only slightly curved and shows characteristic orange peel texture from the original Verneuil boule surface. Natural star sapphires are typically cut with a domed back. Curved growth lines (Verneuil striae) are visible under magnification with crossed polars in most Linde stones.

The star itself is typically sharper, more uniformly bright, and more perfectly six-pointed in Linde stones than in most natural star sapphires, where natural rutile distribution is rarely so regular. The star centre coincides exactly with the apex of the cabochon in Linde stones; natural stones often show offset star centres reflecting irregular crystallographic orientation. UV fluorescence patterns and inclusion suites also differ from natural origin.

Trade and historical significance

Linde star sapphires were widely sold through the 1950s, 1960s, and 1970s as affordable alternatives to natural star sapphires, often in distinctive star-set rings and pendants. The line is particularly associated with US mid-century jewellery production, and Linde-marked stones are common in costume and lower-end fine jewellery from this period. Subsequent Korean, Japanese, and Chinese production has supplied similar synthetic star corundum at lower cost, but the Linde name remains the trade reference.