corundum, synthetic corundum.7, gems

In the Lab Notes section of the Summer 1987 issue of Gems & Gemology, we reported on unusually strong curved color banding in a synthetic yellow sapphire. More recently, the East Coast laboratory had the opportunity to examine two synthetic yellow sapphires that were unusual in other ways. When no color zoning or inclusions are present in a yellow sapphire, it is standard procedure to use immersion, magnification, and crossed polarizers to look for Plato lines as proof of synthesis. One of the two stones we examined displayed such strong Plato lines that they were actually visible without magnification or immersion (figure 15). In contrast, the second stone showed no discernible Plato lines at all, which is unusual for a flame-fusion synthetic yellow sapphire. This stone was proved to be synthetic when high magnification revealed the presence of several small gas bubbles. An interesting blue synthetic sapphire, also examined in New York, showed two sets of curved color banding that intersected each other at a very shallow angle. This is clearly indicated in figure 16 by the wedge of blue with its point almost dead center in the photo. The wedge marks the boundary between the two sets of banding, which are probably a result of changes in the growth conditions while the boule was being formed.