Gem FAQS > Gemstone & Jewelry News & Updates > October 2010- Synthetic Spinel and Synthetic Ruby Doublet

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At top, in reflected light, the separation plane between crown and pavilion is obvious. Below, when immersed in water, the doublet displays different colors in the crown and pavilion, and an abrupt termination of the fracture at the crown. Photos by Alethea Inns, top photo magnified 40x.
Synthetic Spinel and Synthetic Ruby Doublet
The Carlsbad lab has seen a number of corundum doublets over the years, most with natural corundum crowns and synthetic corundum pavilions. We have also encountered colorless spinel crowns attached to a variety of materials.
We were recently asked to identify a 4.85 ct red specimen. Standard gemological testing produced refractive indices of 1.725 on the crown and 1.760-1.770 on the pavilion. Viewed face-up with magnification, the sample revealed a single plane of transparent, colorless glue with planar gas bubbles. The glue layer separated a blue crown from a red pavilion. The separation plane was easily visible in reflected light and indicated an assembled stone.
The crown showed a slightly different luster from the pavilion when viewed with reflected light. Combined with the properties mentioned above, the curved striae indicated synthetic spinel. The pavilion had a large fracture that terminated at the crown, making the doublet even more apparent. Also evident in the pavilion were gas bubbles and curved striae, both typical of flame-fusion synthetic ruby. When viewed with immersion, the two components of the doublet were obvious.
The two components also reacted differently to UV radiation. The crown was inert, while the pavilion showed moderate red fluorescence when exposed to long-wave UV radiation. With short-wave UV, the crown showed moderate chalky green fluorescence, typical of synthetic spinel, while the pavilion had a weak-to-moderate red reaction.
It was unclear what gem this doublet was intended to imitate -- perhaps red spinel. We were surprised to see a doublet with a synthetic crown and pavilion, since it is unlikely to pass for a natural material.

- Alethea Inns
GIA Laboratory, Carlsbad

Last updated on January 16, 2012 by Francis M Lynch