PHOTOLUMINESCENCE FEATURES OF NATURAL PINK DIAMONDS

Natural pink diamonds are renowned for their beauty and rarity. The most common cause of pink color in natural diamond is a 550 nm absorption band that has been correlated with plastic deformation. This absorption feature occurs in both type IIa (i.e., no detectable nitrogen within IR spectra) and type IaAB (i.e., nitrogen aggregates detected within IR spectra) pink diamonds. The pink color in most type IaAB diamonds is concentrated along clearly visible glide planes or grain lines, whereas the pink color is relatively uniform in type IIa stones, despite both being produced by the same absorption feature. Photoluminescence (PL) spectroscopy of pink diamonds colored by the 550 nm absorption band show the presence of a broad emission band spanning ~600–750 nm and centered near 690 nm. The broad emission shows a photochromic effect and spectral oscillations similar to those of the associated 550 nm band, suggesting they may be directly related.

To study this emission band in more detail, we produced PL maps using 455 and 532 nm laser excitation at liquid nitrogen temperature to examine the intensity and peak widths of several PL features often seen in pink diamonds along with this broad emission band. Here we performed PL mapping on type IIa, type IaA<B, and type IaA>B diamonds in order to observe the differences between the colored lamellae and the remainder of the diamond in type IaA>B diamonds, the appearance of the wavy pink graining in type IaA<B diamonds, and the color uniformity of type IIa diamonds. Additionally, we have investigated low nitrogen pink diamonds in order to study in greater detail this transition from the uniform color distribution seen in type IIa pink diamonds to the segregated color centers seen in type Ia pink diamonds.

The PL mapping showed that the spatial distribution of the ~600–750 nm emission band is uniformly distributed within type IIa diamonds, as with the distribution of the pink color. For higher nitrogen type Ia diamonds (> 100 ppm), the presence of the emission band correlates with the concentrated color distribution of type IaAB diamonds. For low nitrogen diamonds, we see a mixture of features with some diamonds showing a combination of features of those seen in the uniform color distribution of type IIa pinks and the sharply defined, distinct and parallel lamellae seen in the type IaA>B pinks.