The HPHT treatment is quite a novel technique that is most difficult to reveal.

   Photoluminescence (excited by a 325 nm laser) spectra of HPHT-treated diamonds, measured at liquid nitrogen temperature, differ from that of non-treated diamonds by the following features: the intensity of 491 nm line considerably decreases (down to complete disappearance of this line), the intensity of 741 nm band decreases as well.

   HPHT treatment of type Ia diamonds modifies their optical absorption spectra as follows: the intensity of 415 nm line increases, while some new lines arise at 503 (H3 centers), 637, and 985 nm (H2 centers). The combination of these lines determines the yellowish green color of the HPHT-treated diamonds of type Ia.

   The HPHT treatment causes defects contained in the diamond to undergo some transformations: simultaneously with aggregation of the defects, the inverse process may also take place. The depth of this process is determined by the original structure of the diamond, as well as by the temperature, pressure, and duration of the HPHT treatment. Non-treated type Ia diamonds have brown color induced by internal plastic strains. The treatment removes these strains (completely or partially) and forms new color centers, changing the diamond color, as a result.

   Thus, the main methods used in diagnostics of HPHT-treated diamonds are absorption spectroscopy and luminescence spectroscopy, and the key diagnostic features are the 503, 637 and 985 nm lines, observed in the measured spectra.