Microwave Spectroscopy as a Potential Tool for Color Grading of Diamonds


  Yossi Rabinowitz  ,  Asher Yahalom  ,  Haim Cohen  ,  Yosef Pinhasi  
Software team leader engineer and electrical engineering PhD student

 The color grading is a dominant property that determines a diamond's value. The color quality is dependent on the light transmittance through the diamond and is influenced mostly by the nitrogen contamination which induces a yellow/brown tint within the diamond. In a small portion of the diamonds a yellow/brown tint is due to structural defects in the crystal. In very rare cases boron contamination results in a bluish color.

 

 Generally, spectroscopic instrumentation (in the infra-red or uv-visible spectral range) measuring polished and rough diamonds are used in the industry to determine a diamonds color and value. However, the results are not accurate enough. Thus there is a motivation for characterization of the diamonds color in longer wavelength, such as microwave. For this purpose, several waveguides which cover frequency range of 3.95-26.5 Ghz, a polyethylene container to hold the diamonds to be studied, as well as suitable resonator mirrors have been produced. Transmission/reflection and resonator methods were used. A good correlation between the S12 parameter and the nitrogen content had been found using the transmission/reflection method. Thus, it is concluded that electromagnetic properties measurements of diamonds in the microwave frequency range can be used to determine their color grading. A very good correlation is obtained between the microwave technique results and results obtained from the infra-red spectra of diamonds .