High-Pressure, High-Temperature or HPHT is a permanent treatment in which heat is used on natural brown diamonds in order to improve their quality of color. Almost all stones possess some degree of yellow or brown color in them, and the HPHT process can reduce the appearance of color in the stone or enhance it. The colors produced by stones that have undergone the HPHT process include clear white, yellow, orange, brown, green and blue.
The HPHT method is a popular treatment due in large part to its relatively low cost. The HPHT technique can only be applied to stones that are categorized as either Type I or Type II. Type I stones contain nitrogen and will turn yellow after the HPHT process, while the Type II variety are naturally nitrogen-free and will render the stones colorless.
The HPHT method utilizes a belt press, a cubic press, and a split-sphere (BARS) press in order to supply the temperature and pressure necessary to successfully change a stone's color. The belt press, which was invented by H. Tracy Hall from General Electric makes use of pressure and temperature amounting up to 50,000 and 70,000 atmospheres. Here a carbon source, a cylindrical capsule and a metal slug all come together as the ideal pressure-temperature is achieved.
Additionally, the Cuban press was developed to increase the pressurized volume created by the belt press. The entire HPHT process can take hours, and in the process, tiny diamond bits are produced. While these tiny stones are too small to be used as gemstones, they are extremely useful for industrial or technological applications. Due to their extreme durability, these tiny stones play a formidable part in cutting, drilling, grinding and polishing a wide range of materials.
Stones that have undergone the HPHT treatment are used for industrial purposes. In this regard, they are more popular than natural stones due to the fact that their mechanical properties are more easily reproduced.
Stones that have undergone the HPHT process are also used and sold as gemstones or for jewelery. They are available in colorless, yellow and blue. Yellow HPHT stones are a result of nitrogen impurities that occur during the manufacturing process. while boron causes some HPHT stones to appear blue. Rarer colors such as pink or green are sometimes produced after HPHT stones are synthesized with the use of irradiation techniques.
Similar to laser drilling and fracture filling, diamond laboratories like require traders to distinguish when a stone has undergone HPHT treatment when they are submitted for grading procedures. However, these laboratories also possess the technology necessary to detect HPHT treatment.
The range of stones subjected to HPHT has increased in recent years and the process for identifying these stones can vary in complexity. Type I HPHT-treated diamonds can be identified based on pitting and burn marks; green fluorescence in visible light; inclusions surrounded by cracks, and graphite at its fractures. Detecting HPHT treatment in Type II diamonds is a much more difficult task.
For anyone in the market for diamonds that have undergone the HPHT process, it is important to be aware that some sellers may claim that their stones have undergone the aforementioned process, even if it is untrue. The HPHT treatment is a very complex and sophisticated process that only a few gemological laboratories in the world can successfully achieve. Some terms that are commonly used by sellers attempting to sell inauthentic HPHT stones include 'ion fused' and 'enhanced' diamonds.
by Thomas J Lee