EFFECT OF LPHT TREATMENT ON NATURAL TYPE IA DIAMONDS

Within the gem diamond industry, type Ia natural diamonds are currently subjected to high-temperature, high-pressure (HPHT) annealing to render low-value diamonds as more saleable or natural diamonds are HPHT treated as the first stage in a multi-step treatment that includes irradiation and moderate temperature annealing at atmospheric pressures.

As a new treatment option, it is unknown the effect of low-pressure, high-temperature (LPHT) treatment on type Ia diamonds and the resulting changes in their spectra and other identifying characteristics. LPHT annealing occurs at temperatures (1600-2200^oC) roughly comparable to those of HPHT annealing, but at much lower pressures (approximately one-half atmosphere compared to 50,000 atmospheres). This much lower pressure makes LPHT annealing much more economically viable and more widely available as a diamond treatment option.

This research tested a wide variety of natural type Ia diamonds to gauge if LPHT annealing created results analogous to HPHT annealing. The study is gauging the effect of LPHT annealing on the color of light, moderate, and dark brown diamonds and on diamonds with varying aggregation state and total nitrogen concentration.

Since type Ia diamonds have the largest natural abundance, this can have significant ramifications within the trade and laboratory testing. The results indicate that LPHT annealing causes significant graphitization, even in well-controlled reducing environments, which limits the annealing time and significant commercial viability.