Lab Notes Gems & Gemology, Winter 2016, Vol. 52, No. 4

Synthetic Sapphire and Synthetic Spinel Doublets

Greenish yellow doublets.
Figure 1. These two doublets, a 6.45 ct greenish yellow oval mixed cut and a 4.17 ct greenish yellow cushion mixed cut, contained a synthetic spinel crown and a synthetic sapphire pavilion. Photo by Robison McMurtry.

Assemblages have been used to imitate various gemstones for many years. Some of the most common are garnet and glass doublets, sapphire and synthetic corundum doublets, synthetic spinel triplets, and beryl triplets.

The Carlsbad laboratory recently examined two uncommon doublets: a 6.45 ct greenish yellow oval mixed cut and a 4.17 ct greenish yellow cushion mixed cut (figure 1). Initial microscopic observation revealed a separation plane near both girdles with flattened, trapped gas bubbles in colorless cement (figure 2, center). The green crowns were joined to the yellow pavilions with this colorless cement (figure 2, left). An RI of 1.728 and whitish chalky fluorescence under short-wave UV on both crowns were suggestive of synthetic spinel. One of the doublets showed thick green curved color banding. Photoluminescence (PL) emission spectra identified the crowns as synthetic spinel.

Photomicrographs of doublets
Figure 2. Left: The green crown and yellow pavilions of the 6.45 ct oval mixed cut are easily observed in immersion. Center: A separation plane near the girdle with a flattened, trapped gas bubble; field of view 1.76 mm. Right: Parallel twining planes in the pavilion; field of view 2.90 mm. Photomicrographs by Nathan Renfro.

In both doublets, the pavilion had a refractive index of 1.760 to 1.768 and was inert to UV radiation. The two assemblages were clean and showed only twining planes (figure 2, right). Faint yellow curved color banding was visible when they were immersed in methylene iodide and observed with a blue color filter. EDXRF analysis showed a chemical composition consistent with synthetic corundum. They contained Ni as a trace element and no Fe, Ga, or Ti.

Microscopic observation and advanced gemological testing confirmed that these were doublets consisting of a synthetic spinel crown and a synthetic sapphire pavilion, joined together with a colorless cement. These are the first assemblages of synthetic spinel and synthetic sapphire observed by GIA.

Najmeh Anjomani is a staff gemologist at GIA’s lab in Carlsbad, California.