COMPARISON OF SPECTROSCOPIC FEATURES IN TYPE IIA AND IA NATURAL PINK DIAMONDS

Natural pink diamonds are easily appreciated for their attractive colors, but not much is known about their origin or the origin of their color. There is an association with the pink color and plastic deformation, which does provide a clue to the formation, but there is still no accepted mechanism of color center formation. The color center responsible for pink does seem to be independent of nitrogen content and thus pink diamonds can be found with nitrogen varying from none to several hundred ppm [1]. In this work, we present a detailed spectroscopic study of pink color in nitrogen-bearing type Ia diamonds and nitrogen-poor type IIa diamonds, as well as gemological comparison between these different diamond types.

The main UV-Visible spectroscopic feature for pink diamonds is the 550 nm band, a broad band responsible for the color. Around 600 nm there can be seen oscillations that are consistent in position and has been previously correlated with pink lamellae [2]. A broad emission band seen in PL spectra of pink diamonds also showed a consistent series of oscillations. PL mapping revealed that the emission band correlates with the pink lamellae. When the spectra are overlaid with a mirror plane around 611 nm, the oscillations are evenly spaced apart suggesting these two bands are directly related.

In the category of type Ia diamonds there are two main nitrogen aggregate forms, the A-aggregate (a substitutional nitrogen pair) and the B-aggregate (four nitrogen atoms surrounding a vacancy). We show that there are distinct differences in the gemological and spectroscopic characteristics in pink diamonds with concentrations of A-nitrogen greater than B-nitrogen (A>B) and the reverse (B>A). For example, the A>B group tend to show tend to show much more pronounced separations between the colored lamellae and the non-colored regions as well as having narrower lamellae on average. A further contrast is with the type IIa group, which tend to show very even color. The PL mapping of various defects showed similar patterns among the different type groups. Most defects showed an increased intensity in the pink lamellae.

[1] S. Eaton-Magana, T. Ardon, K.V. Smit, C.M. Breeding, J.E. Shigley, Natural-Color Pink, Purple, Red, and Brown Diamonds: Band of Many Colors, Gems and Gemology, 54 (2018) 352-377

[2] E. Gaillou, J.E. Post, N.D. Bassim, A.M. Zaitsev, T. Rose, M.D. Fries, R.M. Stroud, A. Steele, J.E. Butler, Spectroscopic and microscopic characterizations of color lamellae in natural pink diamonds, Diamond and Related Materials, 19 (2010) 1207-1220.