NATURAL PINK DIAMOND COLORED BY THE 525 NM ABSORPTION BAND

Two optic defects are normally responsible for natural pink color in diamonds. Most commonly observed is the 550 nm absorption band, associated with plastic deformation, followed by the NV centers, rare in natural-color diamonds but frequently introduced through treatment. This study reports a new color center capable of creating pink coloration in natural diamonds.

A 0.17 carat, type Ia diamond was graded as Fancy Pink in the GIA laboratory. Its color was stable to UV radiation and distributed almost evenly throughout the stone, with one subtle area of dark pink zoning. The sample displayed a unique set of spectra, lacking either of the color centers common to natural pink diamonds. Instead, spectroscopy in the UV-Vis region revealed strong absorption at 525 nm and a broad absorption band centered at ~835 nm. Strong N3 absorption was also detected. Careful spectroscopic analysis did not detect any features that could be linked to color treatment or diamond synthesis. The unusual peaks are associated with high concentrations of hydrogen and nickel within the stone. Preliminary analysis confirms the 525 nm absorption as different from the 520 nm band observed in some CVD lab-grown pink – brown diamonds. It is a new center which, together with the 835 nm band, creates a narrow transmission “window” in the 610-660 nm region. The stone therefore appears pink, instead of yellow – green – grey as is common in stones with similar H and Ni concentrations.

This study revealed a new set of defects that could introduce nice pink color in natural diamonds.