Gemmological and Spectroscopic Characteristics of Black Diamond Treated at High Temperature and Low Pressure

Graphite, as an allotrope of diamond, can undergo mutual transformation with diamond under specific temperature and pressure conditions. Currently, a common treatment for black diamonds in the market utilizes this principle. Graphite inclusions are generated within and on the surface of the diamond under high temperature and low pressure (HTLP) conditions and the abundance of microcrystalline graphite renders the diamond black. Effectively distinguishing natural black diamonds coloured by graphite inclusions from HTLP-treated black diamonds has always been a challenge in laboratory testing.To address this, this study selected 3 natural brown diamond samples for HTLP treatment experiments and 10 HTLP-treated black diamonds. The gemmological and spectroscopic characteristics of the HTLP-treated black diamond samples were analyzed from 4 aspects: the morphology and distribution features of microcrystalline graphite, Raman spectroscopy, diamond luminescence imaging, and photoluminescence (PL) spectroscopy.The results show that the microcrystalline graphite produced by HTLP treatment is primarily distributed on the open fracture surfaces of the treated diamond samples, lacking a specific graphite morphology. Dense arrangements of microcrystalline graphite can be observed on local surface, while pre-existing white cloud-like inclusions expand in volume and turn into black speckles. The Raman spectra of the microcrystalline graphite inclusions mainly exhibit a broad band centered near 1 590 cm^-1. Due to the presence of the graphite D-band, the intrinsic Raman peak of diamond becomes distorted, indicating that the microcrystalline graphite produced by HTLP treatment has a lower degree of ordering than natural graphite in diamonds.The luminescence imaging characteristics of the treated black diamond samples primarily manifest as an overall dark blue fluorescence; fractures exhibit fluorescence inertness. Green luminescence caused by H3 defect can be observed near the fractures, and some samples exhibit a "fluorescence cage" phenomenon. The PL spectra lack the broad emission bands centered at 600 nm and 650 nm, as well as the GR1 defect, while displaying H2 defect at varying intensities.The characteristics of microcrystalline graphite inclusions and their spectroscopic features in HTLP-treated black diamonds can serve as key indicators for their identification.