HPHT is an acronym for a process using High Pressure and High Temperatures. In this high-tech treatment technique, diamonds are subjected to extremely high temperatures and high pressures using similar equipment to that in which diamonds are synthesized. The heat and pressure simulate conditions during a diamond's formation deep in the Earth, and can cause alterations in the diamond's crystal structure and resulting appearance. Whether a color change takes place, and what the change may be, depends on the particular nature of the diamond that is selected for treatment. Some diamonds become much whiter, while others may change to yellow, greenish-yellow, yellowish-green, blue, or pink. Nothing is added during the process, but changes in the diamond's structural arrangement result in the change of color. HPHT is used primarily on higher-clarity diamonds, since inclusions can cause weaknesses in the diamond and lead to severe damage during the treatment process. This treatment is also applied to many synthetically grown diamonds in order to improve their color. The resulting change in color is stable and permanent.
While HPHT diamond treatments are done at extremely high temperature and pressure levels that only diamond can withstand, similar treatments have been developed for other gems using autoclaves. Due to the more delicate nature of ambers and copals, temperatures and pressures are far lower than those used on diamond. Younger copals can be treated to the hardness and durability levels of amber through this “artificial aging” process. Copal or amber can be treated in an autoclave in order to turn them to a peridot-like green color. After treatment, this material is called “greened-amber”, but the starting material may have been either amber or copal. Not all material will turn green, but copals will become harder and more stable, obtaining many of the properties of their much older and more mature cousin, amber. This is a more recent development and research into the exact nature of the transformation is ongoing..