Discrimination of HPHT-treated Type Ⅰa Cape Diamonds Using Optical and Photoluminescence Spectroscopic Techniques

BACKGROUND Spectroscopic characteristics for brown colored Type Ⅱa diamonds treated by high pressure high temperature (HPHT) processes have been investigated extensively. However, almost no reports are available on the variation of the spectroscopic features for the type Ⅰa Cape series brown colored diamonds treated by HPHT processes. OBJECTIVES To help the laboratory to resolve the origin of yellow diamonds. METHODS The ultraviolet-visible (UV-Vis) absorption spectroscopy, Fourier transform infrared spectroscopy (FTIR), and photoluminescence spectroscopy were used to analyze the spectral features for the samples before and after each treatment. RESULTS The color of the diamonds changed from grayish brown to brownish yellow after the HPHT process. Meanwhile, the FTIR/UV-Vis absorption, as well as the photoluminescence was also greatly changed. Besides the 415nm and the 477nm absorption peaks, the treated diamonds also display H3 defect (503.2nm) and the gradually increasing absorption from 550nm to shortwave, resulted in the color changing from grayish brown to brownish yellow. In the mid-infrared region, the three peaks of 1498, 1520 and 1547cm-1 become a broad band centered at 1498cm-1 after treatment, and the photoluminescence spectrum displayed two distinct defects-H3 (503.2nm) and H2 (986.2nm). CONCLUSIONS The spectroscopic features provide a basis for accurate identification of HPHT-treated type Ⅰa Cape series brown diamonds in gem laboratories, and increase understanding of the variation mechanism of the nitrogen and hydrogen related lattice defects in the diamonds during the HPHT processes.