By Ilene Reinitz, Ph.D., GIA GTL manager of Research & Development

Recently, GIA researchers examined diamond color changes that are achieved with heat and pressure. Both natural and treated colors in diamond are caused by various color centers related to impurity elements like nitrogen, by strain within the diamond lattice, and by vacancies, which are produced by either laboratory or natural irradiation. When heat is applied to a diamond (in the laboratory or in nature) these vacancies and impurity atoms migrate through the crystal structure and recombine into different color centers, in chemical transformations that depend on pressure and temperature, and the time that the diamond is held at those conditions. The chemical and structural details of the starting material, and the exact exposure to heat and pressure determine the final color of the diamond.

When a diamond grows in the presence of nitrogen, either in the laboratory or in the earth, single atoms of nitrogen are incorporated into the diamond lattice, forming a color center. This particular impurity is described as a type Ib component, and it causes strong absorption in the blue end of the visible spectrum, sometimes leading to a strong orangy yellow color. In nature, the heat of the earth applied over geological time scales allows these nitrogen atoms to move through the crystal, and form nitrogen aggregates, particularly the A aggregate and the B aggregate, known from infrared spectroscopy. (Another nitrogen aggregate can cause visible absorptions at the N3 and N2 centers, with principal peaks at 415 and 478 nm, yielding "cape" yellow diamonds.) Nitrogen can be aggregated by heating in the laboratory as well, although elevated pressures may be needed to stabilize the diamond and prevent conversion to graphite. Heat also allows vacancies to move through the lattice, and they are frequently trapped at various nitrogen centers. Strain-related graining (often brown in color) can also trap migrating nitrogen and vacancies.

We have had the opportunity to examine samples from three sources: the Super-abrasives division of General Electric Company, Novatek of Provo, Utah, and a group in Sweden (sent to GIA from China). All are currently treating natural brownish diamonds to produce strongly luminescent, greenish yellow to yellow green colors, and also some saturated yellow colors (see Figure 1). The stones are not exposed to laboratory irradiation, although they may contain vacancies from natural irradiation, but the process used to change the colors involves the use of heat and pressure (so called high-pressure-high temperature, or HPHT treatment). We looked at both rough and faceted diamonds, and found that the ones with a greenish appearance have properties much like a group of similarly colored diamonds described in the Gem Trade Lab Notes column of the Summer 1997 issue of Gems & Gemology (p. 136).

These treated diamonds exhibit a strong greenish yellow fluorescence when exposed to long-wave radiation and a similar color but a weaker reaction to short-wave, with some localized chalkiness to both. Mid-infrared spectra show these treated-color diamonds to be type I but without any clear Ib component, containing moderate to high amounts of nitrogen, and typically with more B aggregates than A aggregates. Many of these diamonds show well-defined color zoning, with brown to yellow octahedral graining observed when examined over diffused light. Excitation with a strong light source such as a fiber optic illuminator causes these colored graining planes to luminesce a bright green color; this luminescence strongly contributes to the characteristic overall face-up color of the diamonds.

Typical features observed with a hand-held spectroscope are a weak absorption line at 415 nm, a pair of lines at 503 nm and 495 nm varying in strength from stone to stone, and emission lines at about 513 and 518 nm. The line at 503 is the principal absorption of the H3 center, a vacancy trapped at an A aggregate, and this center produces the green luminescence to both UV and visible light. UV-VIS-NIR spectroscopy corroborated these absorptions, and also showed the H2 peak (at 985 nm, and related to the H3) in all of the samples with a green component to the color. Like the diamonds described in G&G, Summer 1997, a number of these diamonds from each of these sources showed evidence with magnification of high-temperature exposure such as frosted naturals and feathers or faceted girdles with significant bearding.

A most unusual effect was displayed by a 0.51 ct round brilliant synthetic diamond, provided by Ultimate Created Diamond of Golden, Colorado; it showed a distinct color change, from brown greenish-yellow under daylight to orangy-brown under incandescent light. The method used to create this color (and phenomenon) has not been told to us, but the properties we observed indicate the use of heat and pressure. Magnification showed brown graining along cubic directions, forming a long, slim hourglass shape in profile view, and some of this graining luminesced green under strong visible light. Under the LWUV lamp, this same brown graining fluoresced yellowish green, visible through the table as a cross pattern. Under SWUV, the body of the stone fluoresced a weak orange, and the green cross persisted. Unlike previous colored synthetic diamonds we have reported, this one showed a strain pattern (anomalous birefringence) between crossed polarizers that followed the cubic graining.

The infrared spectrum showed A aggregates, rather than the Ib component most common in yellow synthetic diamonds, with well developed H1a and H1b bands (at 1450 and 4935 cm-1 respectively), known to develop in natural diamond following irradiation and heating. The visible spectrum showed a well developed NV center, composed of a single nitrogen atom and a vacancy, with a primary peak at 637 nm, and related absorptions at 594 nm, and 575 nm. The combined absorption of these color centers is responsible for the change-of-color phenomenon. The spectrum also showed an H3 center, the cause of the green luminescence.

This article is based on research funded by donations from Argyle Diamonds and De Beers.

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