Research on Laboratory Testing Features of Chemical Vapor Deposition in Overgrowth Diamonds

BACKGROUNDOvergrowth diamond refers to the product of thick CVD synthetic diamond layer grown on a natural diamond with the whole appearance. Since overgrown diamonds contain information on the nitrogen impurities of natural diamonds, traditional synthetic diamond screening methods and testing procedures are no longer applicable to the detection of overgrown diamonds.

OBJECTIVETo gemologically test an overgrowth diamond sample in order to investigate the full identification features and to propose the optimum detection strategy.

METHODSMinspecting, screening instruments testing, UV-Vis/FTIR/PL spectra examinations, as well as fluorescence and phosphorescence observation under deep UV excitation are carried out.

RESULTSThe sample cannot be identified by traditional microscopic observation, diamond instrument screening, and infrared spectra measurement. The sample has a distinct boundary that is separated by different luminescence under DiamondView^TM. The upper layer displays red fluorescence and greenish blue phosphorescence, while the lower layer shows deep blue fluorescence and no phosphorescence. Infrared spectroscopic analysis shows that the upper layer is type Ⅱa and the lower layer is type Ⅰa diamond. The UV-Vis absorption and photoluminescence spectra confirmed the coexistence of N3 centers and a high level of[Si-V]^- defects in the diamond.

CONCLUSIONSIt was confirmed that the lower part of the sample is natural and the upper part of the sample is CVD synthetic layer with a thickness around 740μm. The first overgrowth diamond reported domestically was compared to the same type of samples reported abroad. The existence of a boundary are missing and the thickness of the CVD layer increased significantly in this sample. The innovative application of the multidirectional luminescence imaging and spectroscopic analyses is the key to laboratory testing of overgrowth diamond.