The Foxfire Diamond, Revisited
The largest gem-quality rough diamond found in Canada, reported earlier in Gems & Gemology (Summer 2016 GNI, pp. 188–189), has revealed remarkable responses to excitation with long- and mid-wave UV light. This 187.63 ct diamond (figure 1) was extracted from the Diavik mine in the Canadian Arctic in the spring of 2015. Aptly named for the aurora borealis, the “Foxfire” displays unusual fluorescence and phosphorescence behavior upon exposure to ultraviolet light. As previously reported, this type Ia diamond has a high concentration of nitrogen impurities, a weak hydrogen-related absorption at 3107 cm–1, and typical “cape” absorption lines.
Exposing this stone to the near-band-gap ultraviolet light of the DiamondView instrument (about 210 nm) and short-wave UV (253.7 nm) results in only very weak fluorescence and phosphorescence, as reported earlier. However, exposure to mid- and long-wave UV (313 nm and 365.0 nm, respectively) produces extremely strong blue fluorescence and strong, long-lived orangish phosphorescence (figure 2).
Another surprising result of exposing the Foxfire diamond to UV light is a noticeable color change from very pale yellow to a light brown color (figure 3). Fortunately, the diamond reverts to its original color in a matter of minutes in ambient room lighting.
We further examined the spectral characteristics of the UV-excited phosphorescence emission with the spectrometer previously used to study phosphorescence from the Hope and other colored diamonds (S. Eaton-Magaña et al., “Fluorescence spectra of colored diamonds using a rapid, mobile spectrometer,” Winter 2007 G&G, pp. 332–351; 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). In the present experiments, the UV light sources employed were mineralogical short-, mid-, and long-wave UV lamps. Figure 4 displays the spectral emission between 350 and 1000 nm (approximately 10 nm resolution) as a function of time after turning off the UV light. The most intense emission resulted from long- and mid-wave UV excitation, while phosphorescence excited by short-wave UV was extremely weak.
The spectra of the “orange” phosphorescence reported above are unusual and distinct from those of other natural and lab-grown phosphorescent diamonds we have examined. We speculate that the primary mechanism for phosphorescence in diamonds is light emission resulting from recombination of electrons trapped at ionized donors and holes trapped at ionized acceptors that are in close proximity to one another. The long time delay results from the thermal movement of the trapped electrons or holes to retrap close enough to one another (P.J. Dean et al., “Intrinsic and extrinsic recombination radiation from natural and synthetic aluminum-doped diamond,” Physical Review, Vol. 140, No. 1A, 1965, p. A352–A386; B. Dischler et al., “Diamond luminescence: Resolved donor-acceptor pair recombination lines,” Diamond and Related Materials, Vol. 3, 1994, pp. 825–830). In the case of the Foxfire, we cannot identify as yet the nature of either the acceptors or donors involved.
Similarly, the phenomenon of the observed color change from light yellow to light brown with UV excitation and the reversal to light yellow has no detailed explanation yet. Such color changes, or sometimes a lightening of color, are often observed in natural diamonds. It is speculated that these are due to charge transfer between various defects (donor or acceptor states) within diamond. Diamond is inherently an insulator where electric charges move very slowly and their motion depends on the nature of the defects present, the temperature of the diamond, and its exposure to light. For an interesting discussion, see K.S. Byrne et al., “Chameleon diamonds: Thermal processes governing luminescence and a model for the color change,” Diamond and Related Materials, Vol. 81, 2018, pp. 45–53.
