HPHT treatment of diamonds
03.05.16
Commercial treatment to change the colour of diamonds began in the late 1940s. This involved exposing the diamonds to radiation and subsequent heat treatment, to foster the alteration of their colour.
High-pressure, high-temperature (HPHT) treatment was introduced to the diamond market in 1999. Originally, it was announced as undetectable; causing great concern in the industry. Today, HPHT enhancement can be recognised by a combination of distinctive spectroscopic and microscopic features.
HPHT treatment is the process of giving a diamond, polished or rough, the second chance; by placing it back into the environment in which it was formed. Extremely high pressure and high temperature causes the diamond to reform, adjust and improve itself. As a result of this process, the colour of the diamond can be dramatically changed.
Scientifically, diamonds are classified into two types. Type I diamonds contain nitrogen; therefore, their colour usually vary from colourless to yellow or brownish. Type II diamonds, which are rare in nature, have only tiny amounts of nitrogen and usually are brown. Type II diamonds that contain boron can also be blue to grey.
In diamonds of the first type, the impurity of nitrogen causes off-brown colours. Through the process of HPHT treatment the platelets of nitrogen can be disseminated within the crystal lattice. This dispersion of nitrogen platelets into small molecules turns the off-colour gemstones into fancy yellow diamonds, making them more sellable.
Diamonds of the second type have tiny amounts of nitrogen to none at all. The reason for off-colour stones under this category is crystal lattice defects that produce brownish hues. HPHT treatment can correct it by again taking the diamond back to its birthplace. The result is a diamond with a restored crystal lattice, higher in quality, value and demand.
While there has been a progress made in detecting HPHT-treated diamonds, such diamonds can still be identified only at experienced and fully equipped gemmological laboratories.
