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
