PREFERENCE IN FORMATION OF H3 OVER H4 AND ITS IMPLICATION IN GEM DIAMOND IDENTIFICATION

Defects H3 and H4 are common in natural, HPHT treated, and artificially irradiated fancy colored diamonds. Understanding of their formation is important for color origin determination in gem labs. However, there are still substantial challenges. Davies (1972) studied the effect of different forms of nitrogen on the annealing of radiation damage, and found A/B = H3/H4. However, Collins (1982, 2001) pointed out that some natural type Ia diamonds could have H3 absorption but even when they contain substantial amounts of nitrogen in the B-form, the H4 absorption is negligible. In this study, based on investigation of a group of 39 (0.12 – 7.03 ct) natural green – yellow diamonds, vacancy source for formation H3 and H4 is identified and preference formation of H3 over H4 is discussed.

Nitrogen concentrations of these diamonds fell in a range of 32 – 496 ppm, with average 0.67 for B/(B+A). So, significant amount of B-form nitrogen exist. No irradiation feature of H1a, H1b, H1c, or 595 nm absorption was detected. Absorption spectra in the UV-Vis region were dominated by defects N3 and H3. Strong emission bands from H3 were observed, and its fluorescence substantially contributed to the body colors of green - yellow. As an outstanding feature, absorption or emission of defect H4 is entirely absent, despite significant concentrations of B-form nitrogen. Strong plastic deformation is a common feature in all samples. Green fluorescence from defect H3 clearly followed dislocation by showing up to 3 sets of parallel lines, instead of following growth zonation. These sharp lines are continuous throughout the whole stones.

Strong dislocations indicated that these diamonds were originlly in brown color after the plastic deformation and thus with significant concentrations of vacancy clusters. Annealed in the earth’s mantle over a long geological history, single vacancies released from vacancy clusters could combine with the A-form nitrogen to form H3. Strong preference in forming H3 over H4 could be related to the unknown disaggregation process of vacancy clusters and the annealing in the earth’s mantle over long geological time. Absence of H4 in this type of diamonds could be considered as a useful indication of natural color origin.