corundum, synthetic corundum.7, twinning, gems

Within a short period of time, the East Coast laboratory encountered two synthetic sapphires that the clients had assumed were natural because of the presence of repeated twinning (figure 13) and what appeared to be boehmite needles contained within the planes (figure 14). Straight twinning lamellae were considered proof of natural origin until 1920, when this feature was noted as an anomaly in synthetic sapphire by Sandmeier and corroborated by W. Plato. For a detailed description of this phenomenon, see the article “Polysynthetic Twinning in Synthetic Corundum,” by W. F. Eppler (Gems & Gemology, Summer 1964). More recently, we reported on the presence of these phenomena in the Summer 1984 (p. 111) and Spring 1989 (p. 38) Lab Notes sections. We conclusively identified the two recent examples described above as synthetic sapphires by the presence of small gas bubbles and curved growth bands, which we observed with magnification and diffused darkfield illumination. The appearance of repeated twinning in a synthetic sapphire can prove to be an identification challenge, especially when the stone in question may have been heat treated. Thermal treatment can diminish curved color banding so that it is only evident when the stone is examined with immersion.