(“Gems & Gemology”, the Quarterly Journal of GIA, at http://www.gia.edu/gemsandgemology/34471/treated_diamond_a_physicist39s_perspective.cfm)
The technologies for the synthesis of diamond via high pressure, high temperature (HPHT) and chemical vapor deposition (CVD) techniques are becoming more refined. The progress is created by scientists and technologists wishing to exploit the remarkable properties of diamond in a wide variety of applications, as well as producing gem-quality synthetic diamonds. Synthetic diamond can be treated, post synthesis, to modify the as-grown properties and to improve performance in some high-tech devices. Also, treatments can change the color of natural and synthetic gem diamond.
In parallel with the developments in diamond synthesis and treatments, the understanding of the defects (both intrinsic and impurity related) that influence the color of natural and synthetic diamond continues to improve. The physics of diamond defect interactions has been extensively studied over the last 30 years, and observing the defects that are created or destroyed through HPHT annealing, irradiation, and combinations of both has contributed to our present understanding of diamond. From this body of knowledge, we have developed the discrimination techniques that can be used in gem laboratories to identify treated diamonds.
In nature, annealing typically occurs at modest temperatures compared to those used in laboratory HPHT annealing treatments. At elevated temperatures, the nitrogen impurity atoms common to most diamonds become mobile and aggregate into defects containing two or more nitrogen atoms. The relatively low temperature allied with geologic time scales ensures that in nature the forward aggregation reaction dominates. However, HPHT treatments (for example to remove brown color) are by necessity carried out over very much shorter time scales (minutes to a few hours), and significantly higher temperatures are required to achieve the desired reaction. The higher temperature also substantially raises the probability of the reverse aggregation reaction (i.e., breaking up nitrogen aggregates). The single substitutional nitrogen defect center (often referred to as the C center) produced in this process is a relatively willing electron donor, and its presence often controls the charge state of other defects. The observation of negatively charged versions of defects (e.g., the negatively charged nitrogen-vacancy defect observed at 637 nm and the negatively charged nitrogen-vacancy-nitrogen defect known as the H2 center) is often key for the identification of treated diamonds. However, it is typically the defect combinations not observed in nature that enable conclusive identification of a treated diamond rather than the spectroscopic identification of a single type of defect.
Mark E. Newton (m.e.newton@warwick.ac.uk)
Department of Physics, University of Warwick, Coventry, United Kingdom
