One of the most important parameters affecting the value of natural colorless diamonds is its light transparency, defined as its color grade. The regular range of color grades is denoted by alphabet letters in the range D-N, where D represents the best quality. The color grade of diamonds is largely influenced by their nitrogen content (when nitrogen atoms substitute for the regular carbon atoms in the crystal).
Since diamonds are also affected by electromagnetic radiation in the GHz frequency range. The electromagnetic properties of several polished diamonds having several nitrogen concentrations in the frequency range of 100-110 GHz (W band) have been studied. The results indicate that there is a good correlation between the amount of nitrogen impurities and the Free Spectral Range (FSR) parameter of a reflection signal (S11) in the waveguide. It can be concluded that measuring the electromagnetic properties of diamonds in order to determine their color grading is feasible and effective. In addition, the FSR measurements were correlated well with the FTIR measurements. A novel color estimate, based on the FSR vs the nitrogen correlation, has been tested on 26 diamonds with a success rate higher than 70%.