The Spectral Characteristics and Identification Techniques for Colorless and Near-colorless HPHT Synthetic Diamonds

Melee-sized colorless HPHT synthetic diamonds mixed with natural diamonds in jewellery have been reported by gem testing laboratory. It was noted that the differences of the fluorescence and phosphorescence between natural and synthetic diamonds are the key features for quick screening in mixed diamonds. However, it is difficult to completely distinguish the two only by the differences of fluorescence and phosphorescence. In this paper, 5 colorless gem-quality HPHT synthetic diamonds were systematically investigated and characterized using mutiple spectroscopic techniques, together with traditional gem testing techniques. It was found that 5 samples display no, or weak, 270 nm absorption band at the UV-Vis spectra, and the lower the color grade, the stronger the 270 nm absorption band. Infrared absorption spectra showed that all samples contain boron. The photoluminescence spectra revealed that the colorless HPHT synthetic diamonds contain N, Ni or Si-related lattice defects. Under irradiation of the ultra-short wavelength light, all samples phosphoresced strongly with blue-green color. Typical octahedral and cubic growth patterns can be clearly observed under the DiamondView^TM imaging system. Although there are some differences between the samples, the HPHT synthetic diamonds can be accurately identified through careful analysis combining all features, particularly the spectral charateristics of phosphorescence and fluorescence, FTIR, UV-Vis, and photoluminescence spectra.