Abstract: The colour of diamond is an important quality parameter. Brown type Ⅱa diamonds can be upgraded to higher colour grades (D-H) through high pressure and high temperature (HPHT) treatment, significantly enhancing their commercial value. This treatment needs to be disclosed in the trade. Currently, spectroscopic techniques have become the primary method for detecting HPHT-treated colourless type Ⅱa diamonds by analyzing thermally unstable defect centers. The research demonstrates that the unstable GR1 center (741 nm) is of great significance in identifying this kind of treated diamonds. However, since GR1 center can be obtained by both natural and artificial irradiation, it is essential to verify its origin as non-treatment-related secondary irradiation products.In this study, HPHT-treated colourless type Ⅱa diamond sample was artificially irradiated and annealed at different temperatures (800℃/ 900℃), and its gemmological and spectroscopic characteristics after treatments above were studied using UV-Vis spectroscopy and photoluminescence spectroscopy. The results indicated that the GR1 center plays a pivotal role in the identification of natural and HPHT-treated colourless type Ⅱa diamonds.The presence or absence of GR1 center along with its relatively narrow full width at half maximum (FWHM≤0.60 nm, tested under 532 nm excitation) can distinguish natural and HPHT-treated colourless type Ⅱa diamonds.