Gem News International Gems & Gemology, Winter 2014, Vol. 50, No. 4

Unusual Short-Wave UV Reaction in Synthetic Blue Spinel

Pragati Verma

具有异常短波紫外线反应，用焰熔法生长的合成尖晶石
Figure 1. This 1.44 ct synthetic spinel was grown by the Verneuil process but displayed the short-wave UV reaction of a flux-grown sample. Photo by Pragati Verma.
The Indian Gemmological Institute – Gem Testing Laboratory recently examined a 1.44 ct blue oval mixed cut specimen (figure 1). Standard gemological testing revealed the following properties: RI—1.728; hydrostatic SG—3.64; strong anomalous double refraction under the polariscope; Chelsea filter reaction—strong red; and red transmission in fiber-optic light. The sample showed general absorption bands between 535–550, 560–590, and 615–635 nm in the handheld spectroscope. Microscopic examination with a fiber-optic light source revealed shiny dotted inclusions that were scattered and created a wavy pattern; in some areas, the dotted inclusions were arranged in a series (figure 2). This wavy pattern has not been reported in natural spinel. The author has seen this pattern in some Verneuil synthetic spinel. There is a possibility that these inclusions might be very minute gas bubbles or unmelted feed powder.

Dotted inclusions in synthetic spinel
Figure 2. Series of dotted inclusions were seen in some areas of the synthetic spinel.
The dots were so concentrated that it was difficult to distinguish them individually.
Photomicrograph by Pragati Verma; magnified 25×.
These properties—higher RI, higher SG, red Chelsea filter reaction, anomalous double refraction, and absorption spectrum features—are diagnostic features of cobalt-doped synthetic blue spinel grown by the Verneuil (flame-fusion) process (M. O’ Donoghue, Synthetic, Imitation & Treated Gemstones, Butterworth-Heinemann, Oxford, UK, 1997, p. 153).
 
When tested with UV, this synthetic spinel had an unusual reaction. Under both long- and short-wave UV, the sample showed red fluorescence (figure 3). This was an unexpected short-wave UV reaction for Verneuil-grown blue synthetic spinel, which generally shows a mottled blue to bluish white reaction. This kind of fluorescence was observed in synthetic blue spinel grown by a flux method (S. Muhlmeister et al., “Flux-grown synthetic red and blue spinels from Russia,” Summer 1993 G&G, pp. 81–98).

Short-wave UV reaction of synthetic spinel
Figure 3. The synthetic spinel displayed red fluorescence under short-wave UV. Photo by Pragati Verma.

Pragati Verma is an executive gemologist with the Indian Gemmological Institute – Gem Testing Laboratory in New Delhi.