Lab Notes Gems & Gemology, Summer 2020, Vol. 56, No. 2

HPHT Laboratory-Grown Pink Diamond with Unstable Color

Kaitlyn Mack and Paul Johnson

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HPHT-grown synthetic diamond with unstable pink color.
Figure 1. This 3.02 ct HPHT-grown diamond with color equivalent to Fancy Vivid pink had an unstable color. Photo by Jian Xin (Jae) Liao.

While rare, it is not uncommon for certain types of diamond to change color when exposed to different lighting or thermal conditions. Consider, for example, chameleon-type diamonds, which can change color from yellowish green to orangy yellow when heated or concealed in the dark for long periods of time.

Strong color zoning (left) and fluorescence (right) seen in an HPHT-grown synthetic diamond.
 
 
Figure 2. Left: Strong color zoning observed under natural lighting conditions. Right: The fluorescence pattern follows this zoning, as observed in the DiamondView image (strong short-wave UV). Image by Paul Johnson (right).

Recently submitted to GIA’s New York laboratory for examination was a 3.02 ct laboratory-grown diamond with the equivalent color grade of Fancy Vivid pink (figure 1). Typical of an HPHT-grown laboratory-grown diamond, it exhibited strong color zoning (figure 2, left) related to its typical hourglass synthetic growth structure. Of note, it exhibited very strong orange fluorescence when exposed to ultra-short-wave ultraviolet radiation (figure 2, right). This is of note as it is indicative of post-growth treatment to create the desirable pink color.

Photoluminescence spectrum showing strong nitrogen vacancy centers.
 
Figure 3. 514 nm (green) photoluminescence spectrum showing strong NV (nitrogen vacancy) centers at 575 and 637 nm. These are responsible for the very strong bodycolor and orange-red fluorescence.

This diamond was grown with a very carefully controlled nitrogen content that produced approximately 1.6 ppm of type Ib single substituted nitrogen (post treatment). The nitrogen content was calculated from a normalized mid-infrared spectrum. Post-growth treatment of irradiation/annealing generated very strong absorption from the nitrogen vacancy (NV) centers that are responsible for both the pink bodycolor and the very strong orange-red fluorescence (see the PL spectrum in figure 3).

HPHT-grown synthetic diamond experiences temporary color change when exposed to strong short-wave UV radiation.
Figure 4. A dramatic hue shift from pink to orange was observed when the synthetic diamond was exposed to short-wave UV radiation. Color swatches (bottom) were generated from recorded LCH values both before and after UV bleaching. The two images (top) were taken using standard lighting in the GIA colorimeter, which recorded the LCH values.

When excited by short-wave UV radiation, the nitrogen vacancy color-causing centers are bleached and desaturated, resulting in a hue shift to orange (figure 4). This is a temporary color change, and within minutes the diamond will revert to its stable pink color under normal daylight conditions.

Using LCH (lightness, chroma, and hue) values, color swatches (figure 4, bottom) were generated that accurately represent the color change observed. The images in figure 4 (top) were captured using a GIA-built colorimeter with standard lighting conditions.

With the recent influx of laboratory-grown fancy-color diamond into the market, these phenomena are of note to consumers.

Kaitlyn Mack is staff gemologist, and Paul Johnson is manager of analytics, at GIA in New York.