LARGE NATURALLY GROWN DIAMONDS WITH TREATED PINK COLOR

Pink diamonds are prized for their rarity, with the majority colored by a broad absorption band at ~550 nm associated with plastic deformation. The color is often concentrated within lamellae oriented along {111} glide planes. Alternatively, pink diamonds may be colored by nitrogen-vacancy (NV) centers with zero phonon lines at 575 and 637 nm for the neutral and negative charge states, respectively. This latter group, often nicknamed “Golconda diamonds,” only accounts for ~0.6% of natural pink diamonds submitted to GIA. The most famous is the 34.65 ct Fancy Intense pink Princie Diamond, which was sold for $39 million in 2013. We examined four Fancy Vivid pink-colored diamonds weighing 15.21, 20.96, 33.07 and 55.55 cts. We confirmed that they are naturally occurring diamonds, but their pink color was found to be the result of multi-step treatment to produce NV centers.

FTIR spectra revealed that they are type IIa and type Ib diamonds. These types of diamonds can possess vacancy clusters which produce an unattractive brown color. Spectral evidence suggests that the first step in the treatment sequence was annealing at high pressure and high temperature (HPHT), which is known to break apart the vacancy clusters and reduce the brown color. HPHT treatment created H2 (NVN^-) centers which can be observed in photoluminescence spectra. Then, they were artificially irradiated to generate vacancy centers. Finally, these diamonds were annealed at lower temperatures (> 600 °C) so that vacancies became mobile, migrating till they were trapped at nitrogen atoms to form NV centers. Strong absorption at 637 nm (NV^-) and its side bands in UV-Visible spectra confirm they are responsible for their saturated pink color.

Multi-treatment for pink color production has been reported since the 1950s, yet the 33.07 and 55.55 cts stones mark the largest treated pink diamonds submitted to GIA. They also highlight the boldness of treaters as the original diamonds – irrespective of color – would have considerable value due to their size. Since the 2020 closure of the Argyle mine, the largest supplier of natural pink diamonds, it is possible that the proportion of notable treated pink, mined or laboratory-grown, diamonds may increase to meet demand.