A model of HPHT color enhancement mechanism in natural gray diamonds

HPHT-Treated Diamonds

“…It is believed that the two major sources of gray color are graphitic micro-inclusions and hydrogen (Vins and Kononov 2003). Since perfectly uniform absorption in a wide spectral range occurs rarely, gray color of natural diamonds is usually accompanied by faint tints of yellow, green, blue or pink.…”

“…For instance, type I diamonds from the kimberlite pipe ''Mir'', grew at temperatures of 1,400-1,450°C under pressure of 4-6 GPa (Vins and Kononov 2003). Majority of natural diamonds were formed in subcontinental lithosphere at depths of 180-200 km, where temperature and pressure could vary from 900 to 1,400°C and 4-7 GPa respectively.…”

“…Type IaA diamonds of initial gray color can be also converted into yellow by HPHT treatment at temperatures from 1,800 to 2,100°C (Vins and Kononov 2003). Green color modifier is generated in initially brown type Ia diamonds only and only in combination with yellow color.…”

“…However, if the treatment temperature is 2,100°C and above, or if the nitrogen content is high, the treated diamonds acquire yellow color (Vins and Kononov 2003). However, if the treatment temperature is 2,100°C and above, or if the nitrogen content is high, the treated diamonds acquire yellow color (Vins and Kononov 2003).…”

Spectroscopic Characteristics of Treated-Color Natural Diamonds

“…Generally, the gemologists have begun to enhance the color of diamond since the 1930s [2] and diamond treatments in the modern era mostly involve high-energy irradiation, high pressure and high temperature (HPHT) treatment, low pressure and high temperature (LPHT) annealing, and combination of these treatments [3][4][5][6][7][8][9][10][11]. Such as Vins [12] reported that natural diamonds with brown hue could be altered to display purple to red color, through multiprocess treatments including HPHT treatment (6-7 GPa at over 2150°C), high-energy irradiation, and LPHT annealing (lower than 1100°C).…”