Most natural diamonds have yellowish or brownish tints which significantly affect their value. That is why the diamond trade has always sought ways to improve undesirable diamonds by enhancing their colour. Let’s see what are colour enhanced diamonds, and what types of diamond colour treatments and enhancements there are.
What Are Colour Enhanced Diamonds?
Since colourless diamonds are more valuable than tinted stones and deeply saturated fancy colour diamonds are more expensive than lighter ones, some companies have experimented with artificially enhancing and altering the diamond colour through different methods.
There are three main methods used to artificially enhance the colour of a diamond: HPHT (high pressure, high temperature), coating and irradiation.
Diamond colour treatments are acceptable within the industry as long as they are explicitly disclosed to the customer. In fact, the origin of the diamond colour, whether natural or artificially enhanced, must be noted in the grading report.
High-Pressure, High-Temperature (HPHT)
HPHT colour-enhancing treatment uses high pressure and high temperature machines that are the same as those used to grow synthetic diamonds. This treatment is effective for enhancing the colour of some rough or polished stones, turning them into colourless, yellow, orange-yellow, yellowish-green, green, pink and blue.
HPHT treatment is mostly applied to Type IIa diamonds which are colourless but because of the distortion of their crystal lattice show brown colour. Exposing the diamond to high pressure and high temperature removes the brown colour by reducing the structural irregularities and transforms the stones into near colourless and colourless diamonds, rarely into pink and blue. Type Ia brownish diamonds can be transformed to fancy yellow, fancy yellowish-green and green colour; however, colour-enhanced fancy yellowish-green and green diamonds are usually easily detectable due to their strong fluorescence.
HPHT is a permanent treatment, meaning the colour of the diamond does not revert to its original state with time. Depending on the stone, the colour change can be minor or dramatic.
Since this form of colour enhancement is virtually undetectable outside of a well-equipped grading laboratory, the Federal Trade Commission (FTC) requires it to be explicitly disclosed. For example, grading reports issued by the Gemological Institute of America (GIA) always contain a note about HPHT treatment if detected. In some cases, treated diamonds may also have a laser inscription indicating the treatment.
As the name suggests, this colour-enhancing treatment uses an application of an overlay coating which often contains fluoride, silicon dioxide, aluminium oxide, or titanium oxide to alter or mask an undesirable body colour.
The “dye” can be applied both to the whole diamond and to the parts such as the pavilion or girdle to create the illusion of uniform colour distribution. For example, silicon dioxide coating applied to polished near colourless and colourless diamonds can produce a variety of natural-looking fancy colours, such as pink, purple, yellow, orange and blue.
The coating is a nearly undetectable temporary colour treatment, meaning the colour is reversible and can wear or be scratched from the surface of the diamond. Being a fairly durable treatment, the coating can easily be damaged by heat and chemicals during jewellery repairs and polishing.
This treatment method is also used to make fake diamonds by applying a thin film of synthetic diamond to the surface of a diamond simulant to give it certain characteristics of a real stone. In case such treatment is not disclosed, it is a scam.
Irradiation and Annealing
Irradiation is a colour-enhancing diamond treatment used to turn colourless diamonds into affordable fancy colour diamonds without causing any damage to the stones.
The process of irradiation involves the use of high energy particles, which alter or damage the crystal lattice of a diamond. This alternation creates many “colour centres” throughout the stone resulting in a change of colour in the diamond. The most common colours produced by irradiation are different shades of blue and green.
Sometimes, irradiation can be followed by annealing, which is a controlled heating and cooling process to change the colour of irradiated blue or green diamonds to yellow, orange, brown, pink, red and purple. When used by itself, annealing can alter the colour of the stone to various shades of blue, green, yellow and brown, depending on the colour of the starting material.
The result of irradiation and annealing is permanent under normal conditions, however, such stones are sensitive to heat and jewellery repair procedures, recutting and repolishing as they can drastically change their colour.