Characterization of Diamond Colors via Microwave Spectroscopy

The color grading of diamonds is affected mainly by the mode/concentration of nitrogen contamination in the diamond and to much lesser extent by structural defects in the crystal, although in very rare cases boron contamination also causes the appearance of a bluish grayish color. The characterization of rough diamonds for the jewelry industry is usually carried out by spectroscopic instrumentation (in the UV-visible or the Infra-red spectral range) or by XRD/surface techniques. However, the electromagnetic properties of diamonds in longer wave lengths can also shed light on their composition and structure and are much less sensitive to surface effects. The electromagnetic properties of several polished diamonds with different nitrogen concentrations have been determined. Some waveguides in the frequency range of 8-26 GHz and a polyethylene container to contain the diamonds to be studied have been designed and produced. The results have indicated that there is a good correlation between the nitrogen impurity content and the S12 parameter of a transition signal in the waveguide. Thus, it can be concluded that one can use measurements of the electromagnetic properties of diamonds in order to determine their color grading. In addition, a good correlation has been observed with the FTIR measurements of the diamonds.