Winter 2011

Featured Lab Note: Type IIb Diamond with Long Phosphorescence

It is well known that type II synthetic diamonds and some natural diamonds (including type IIb) are phosphorescent. In natural diamonds the effect usually lasts for only a few seconds. However, a 4.23 ct emerald cut recently submitted to the Carlsbad laboratory proved to be an exception. Identified as an untreated type IIb diamond with D color and IF clarity, it displayed very weak to weak greenish blue fluorescence to long- and short-wave UV radiation. During UV exposure, the fluorescence appeared to intensify due to the sample’s strong phosphorescence. After 5 seconds of exposure to short-wave UV radiation, the diamond displayed bright blue phosphorescence (see video in the G&G Data Depository), and it continued to luminesce for several minutes.

Phosphorescence spectroscopy revealed a band at 500 nm (also documented in type IIb diamonds by S. Eaton-Magaña et al., “Luminescence of the Hope diamond and other blue diamonds,” Fall 2006 G&G, pp. 95–96) that was responsible for the blue phosphorescence. At the end of 60 seconds, the 500 nm phosphorescence band was still visible. The calculated half-life of this band was 8.4 seconds. By comparison, the Hope diamond’s 500 nm band half-life is only 1.8 seconds (S. Eaton-Magaña et al., “Using phosphorescence as a fingerprint for the Hope and other blue diamonds,” Geology, Vol. 36, No. 1, 2008, pp. 83–86). The relationship between apparent bodycolor and phosphorescence half-life was examined in 300 natural type IIb diamonds in the GIA laboratory. The present diamond’s 500 nm band had a much longer half-life and a stronger intensity than most natural type IIb diamonds; this combination produced the unusually long phosphorescence.

This diamond also exhibited thermoluminescence, as described in the Spring 2011 Lab Notes (pp. 50–51). It was immersed in a liquid-nitrogen bath (–196°C) and simultaneously exposed to short-wave UV. After removal from the bath, it warmed up rapidly and displayed a flash of blue thermoluminescence. After a few seconds of additional warming, it exhibited the long-lasting phosphorescence described above. A second video documenting both luminescence behaviors is available in the G&G Data Depository.

Andy H. Shen and Sally Eaton-Magaña