LUMINESCENCE INVESTIGATIONS OF NATURAL COLOR-CHANGING DIAMONDS

Some diamonds reversibly change color in response to the influence of temperature and light. An understanding of how these processes occur in each unique case has both scientific and commercial impact for the identification and authentication of diamonds.

The underlying physical systems that drive these diamond color changes may also manifest changes in other measurable properties, such as luminescence. We have investigated luminescence in two classes of diamond noted for displaying color changes: (1) pink/brown diamonds, and (2) ‘chameleon’ diamonds. The defect centers responsible for diamond color in these cases are not well understood; we examined luminescence behaviors seeking to learn more about electronic transitions underlying the diamond color, and color-change.

Some pink/brown diamonds demonstrate two distinct phases of photochromism – one induced by excitation in the visible domain, and one sensitive to ultraviolet and infrared light. The latter induces luminescence from diamond defects that do not otherwise interact with the pink color center. A new survey shows that this luminescence is visible in a wide range of pink/brown diamonds, and involves a larger number of defects than had been previously observed. We will discuss the implications of these results for the current understanding of pink diamond color, and its relationship with brown color in diamonds.

‘Chameleon’ diamonds show characteristic fluorescence peaking near 560nm. This band is strongly temperature-sensitive, suggesting a possible association with the diamonds’ thermochromism. 560nm-band phosphorescence following UV exposure suggests a thermal release of trapped charge carriers from several distinct traps. Infrared light can also empty the trap states, and may be used to measure lifetimes of long-lived states.