Gem News International Gems & Gemology, Summer 2021, Vol. 57, No. 2

Rare Mixed Type IaB-IIb Diamond with a Long-Lasting Phosphorescence

FTIR spectrum confirming a mixed-type diamond.
Figure 1. The 0.20 ct diamond’s FTIR spectrum. Insets show the type IaB aggregate nitrogen at 1175 and 1010 cm–1 (top inset) and IIb boron (bottom inset) at 3725 and 2803 cm–1, confirming a mixed-type stone.

Phosphorescence is an optical effect in which electromagnetic radiation absorbed by a substance is released relatively slowly in the form of light. Long-lasting UV phosphorescence of more than 10 seconds’ duration is an extremely rare property in natural diamonds, normally limited to hydrogen-rich type Ia chameleon and type IIb diamonds.

Recently, a natural round brilliant weighing 0.20 ct (E-F color, VS2-SI1 clarity) was submitted to Stuller Inc. by a client for a custom design. During our diamond screening process, using a phosphorescence imaging-based screening device, the diamond was automatically identified as having an HPHT-grown origin, conflicting with the client’s natural origin disclosure. Secondary testing, using a fluorescence spectroscopy-based screening device, identified the diamond as having a natural origin. Under magnification, only a few pinpoints were spotted.

Due to the ambiguous results, the gem was sent to Stuller’s Gem Lab for further analysis. The FTIR spectrum (figure 1) showed a mixed diamond type comprising low levels of aggregated nitrogen (IaB) and boron (IIb), indicating a natural origin. Similar results were given using fluorescence spectroscopy, which identified N3 peaks, associated with natural origin. Photoluminescence spectroscopy, using a 532 nm laser under liquid nitrogen temperature (77 K), provided results supporting the other instruments and confirming the absence of synthetic origin-related peaks.

Images showing phosphorescence from 0 to 60 seconds after exposure.
Figure 2. The 0.20 ct diamond’s phosphorescence, as seen immediately after exposure to UV radiation (left) and after 60 seconds of elapsed time (right). Images by Guy Borenstein.

In order to identify the cause of the false positive results from the first screening device, the diamond was placed in a SWUV cabinet to analyze its phosphorescence effect. Once excited, the stone emitted a strong bluish green phosphorescence for about 120 seconds (figure 2).

The emitted color and the unusually long duration of the effect are typically associated with HPHT-grown diamonds. Consequently, these properties triggered the false positive results in the phosphorescence imaging-based screening device. In this case, the rare combination of natural diamond types, which includes a boron component, was probably the reason for the unusual phosphorescence.

This case demonstrates the complexity of diamond origin identification and the need for multiple diamond screening devices, each with a different screening technology, to cover all possibilities and ensure a 100% correct identification.

Guy Borenstein is senior gemologist, and Sean Oneal is D&G procurement specialist, at Stuller Inc. in Lafayette, Louisiana.