Treated diamonds are stones that have been subject to a process that improves their clarity or change their colour, whereby the process is not considered part of the standard cutting and polishing procedure. Fair trading practices require that treated diamonds are disclosed as such, and the treatment or treatments that they have undergone are described, at the point of sale. Certain of the treatments are permanent, but others are likely to be reversed. Common treatments include the following:
 
Coating: This is the oldest way of changing the colour of a diamond. It involves a paint that partly or completely covers the diamond’s surface, thus intensifying or changing the colour. Coating is not permanent and can easily be removed by scratching or deep boiling.


Irradiation and annealing: By exposing a diamond to radiation within a linear nuclear reactor, its colour can be changed permanently. Diamonds that are subject only to irradiation result in green, blue or black colours. When irradiation is followed by annealing, yellow, red or pink colours are obtained, depending on the starting material.
 
High Pressure-High Temperature (HPHT): Permanent colour change is achieved by subjecting diamonds to extremely high pressures and temperatures. Not only can fancy colours be obtained, but also colourless diamonds, with mostly brown diamonds having their colour removed. This method uses HPHT growth presses that are modified in order to reach a temperature of 2,000°C and more, and a pressure of 70,000 atm. The significant temperature is necessary to change the colour of a diamond, while the high pressure prevents any damage to the stone.
 
Fracture-filling: A filled diamond, or fracture-filled diamond is a diamond in which breaks reaching the stone’s surface have been filled with a foreign substance, with the purpose of making the crack less visible and improving the clarity grade of the stone. This process must be disclosed to the buyer when the diamond is sold. This is not a permanent treatment and can be reversed if the diamond is subject to deep boiling.