How to Make Diamonds Change Colour
'One carat could be worth about £10,000-£15,000'
Jamie Brener | 3 December 2017

On the 10th April 2017, Apollo and Artemis, a pair of diamond earrings, one blue, one pink, sold for £44.9 million in Geneva. However, this failed to beat the record of £63.5 million, held by the Pink Star diamond. What these diamonds reaching record prices have in common is that they are all coloured. The reason why the coloured diamonds obtain higher prices over pure white diamonds, is that coloured diamonds are simply rarer.

So, how do diamonds become coloured? Firstly, there are diamonds that obtain their colour through natural processes in their formation. Secondly, diamonds can be made to change colour through “High Pressure, High Temperature, Treatment” known as HPHT. HPHT causes something know as plastic deformation. This is where a fault is made in the crystal lattice (structure) that changes the frequency of light that the structure reflects and therefore changes the colour of the diamond.  

Natural diamonds often have impurities in them which change the frequency of the light they emit, thus changing the colour of the diamond. In natural coloured diamonds, nitrogen impurities cause a yellow or brown colour. Pink, red and brown diamonds that do not  contain nitrogen are coloured due to plastic deformation that happens during growth. Light blue diamonds have scattered boron throughout the crystal lattice. Green diamonds are formed due to exposure to varying radiation. Black and grey diamonds are caused by microscopic inclusions of graphite and Sulphur. Finally, purple diamonds are formed from lattice distortion and a high Hydrogen content.

We can make white diamonds (regular) turn into coloured diamonds through a variety of techniques. One of the sneakiest methods used by dealers is to apply a very thin translucent coating to a poor-quality diamond, that makes it appear whatever colour the dealer desires to sell. However, this coating is not permanent and can be removed just by normal wear and cleaning. The other main way to make a diamond change colour is to ‘irradiate’ it, another way to refer to HPHT. This involves exposing the diamond to radiation before placing the diamond under extreme heat and pressure. Doing this will only turn the diamond green, red, blue, purple, vivid yellow and occasionally pink. This colour is permanent and cannot be removed easily like the coated diamonds.

Most brown and yellow diamonds are not desirable as they are very common and caused by nitrogen impurities, so gemologists came up with ways to remove those colours from the diamond. One technique is to paint dots of blue or purple ink on the diamond to counteract the yellow colour. But once again, like the coating used to colour diamonds, these dots rub off easily and can be removed with water. The other more permanent way to remove yellow and brown colour is HPHT. The same treatment used to turn white diamond different colours can be used by diamond companies to turn undesirable low-grade diamonds into higher-grade diamonds. Companies who perform this treatment often argue that HPHT is not in actuality a treatment but that they are merely finishing what nature started, causing a heated debate around the topic.

It is important to note the difference in value between a natural coloured diamond and a coloured HPHT diamond. A natural type IIa light fancy pink diamond, weighing one carat could be worth about £10,000-£15,000, while a HPHT diamond of the same specification might only be worth £1,500-£2,000. Gem labs such as the Gemological Institute of America (GIA) in New York, Gemmological Certification Services (GCS) in London and GGTL Laboratories in Geneva, test and grade coloured diamonds to see if they are natural or not. And so, if you are ever buying a coloured diamond, make sure to check that it has a certificate from one of the afore mentioned labs or another respected and trusted laboratory.


Image sourced under Creative Commons License

James Routledge 2016