Colour Centres in Diamond

An Update on Color in Gems. Part 3: Colors Caused By Band Gaps and Physical Phenomena

“…However, because this donor level has a finite width, light of a variety of wavelengths extending from the ultraviolet into the visible range up to 560 nm (green) will be absorbed, creating a strong yellow color. This type of coloration occurs only in type Ib diamonds, in which isolated nitrogen atoms substitute for carbon atoms in the proportion of about 1 to 100,000 (Collins, 1982). This color is distinct from the yellow color commonly caused by the nitrogen-related N3 color center, which produces the familiar Cape series of absorption lines.…”

The Legendary Dresden Green Diamond

“…Therefore, not only is the famed Dresden Green the largest known natural green diamond, but it is also a very rare type IIa. This was an unexpected discovery, since type la diamonds represent as much as 95% of natural gem diamonds (Collins, 1982).…”

“…The GR series, ranging from GR1 through GR8, is found in all diamonds that have been subjected to radiation damage, with the GR1 at 741 nm being by far the strongest (Collins, 1982). The TR12 line (TR standing for Type I1 Radiation) is the strongest in a series of lines that are referred to as TR12 through TR17 (470.1,468.8 [TR12A],464.3,446.5,444.7,440.2, and 438.0, respectively).…”

“…The line is associated with the annealing of irradiated diamonds (Dugdale, 1953;Crowningshield, 1957;Collins, 1982;Guo et al, 1986;Fritsch et al, 1988), so it is normally visible in annealed colors (e.g., yellow, brown, orange, pink). It is also occasionally seen in laboratory-treated diamonds that are still green (Kane, 1988;Fritsch et al, 1988).…”

Spectroscopic Characteristics of Treated-Color Natural Diamonds

“…The H3 (503.2 nm) and H2 (986.2 nm) centers are related to the nitrogen-vacancynitrogen (NVN) defect in neutral and negative charge state, respectively. Commonly, the H3 center has been observed in diamonds involving with irradiation and high temperature annealing (or HPHT annealing) [28][29][30] or been directly created in plastically deformed brown diamonds through heating at high temperature [31,32], while various concentrations of the H3 defect can also occur in some natural diamonds [33][34][35][36]. In nitrogen-containing diamonds, highenergy irradiation followed by annealing at temperatures over 500°C could result in the formation of H3 center [26].…”