Abstract
In the present paper HPHT-processing of natural gray graphite containing diamonds is studied and a new model of the color enhancement mechanism is proposed. It is proposed that there occurs a polymorphous transformation of graphite inclusions into diamond during HPHT-processing. During this transformation hydrogen, the content of which depends to a great extent on the number of graphite inclusions, is transferred from graphite to diamond getting localized on the nearest structural defects with uncompensated negative charges. In the optical spectra this is accompanied by an anomalous increase in the intensity of the hydrogen-related lines in the IR spectrum. It has been observed that the intensity increases in the 2.988 eV line attributed to N3 centers after HPHT-processing at an unusually low temperature of 1800 °C. In natural brown diamonds the intensity increase of the 2.988 eV line takes place at higher HPHT temperatures (over 2000 °C).
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