What makes pink diamonds pink has puzzled gemmologists and scientists alike since the precious and highly sought-after gemstones were first dug out of the ground.
Scientists understand that nitrogen atoms in a diamond will make it appear yellow and boron impurities can make a diamond appear blue, but what exactly gives pink diamonds their distinctive hue has been an ongoing conundrum.
However, new research has shed light, literally, on Australia's naturally pink Argyle diamonds. Scientists at the University of Western Australia have taken a closer look into the photochromic behaviour of Argyle diamonds – or the way in which the diamonds change colour when exposed to certain light frequencies.
The study was published recently in the Diamond and Related Materials journal and details the researchers’ use of special light beams to help understand the pink diamonds’ photochromic behaviour. Lights with narrow frequencies were used to bleach the colour from pink diamonds.
“We are attempting to understand this process better – what light removes the colour, what light brings it back, and what this all means for the diamond's structure,” PhD student Keal Byrne said when quoted in National Geographic.
“[We] hope that this may be one step towards finally determining what it is that creates the pink colouration in Argyle diamond.”
Diamonds are made from carbon atoms bonded together in a crystalline lattice that doesn’t absorb light, which results in the colourless appearance of a natural diamond.
Coloured diamonds, such as Argyle diamonds, have what is known as a defect centre, where one or more of the carbon atoms in the diamond lattice may be missing, or may have been replaced with a different element.
Naturally occurring pink diamonds include defect centres with properties that differ greatly from those of commonly synthesised diamond centres. These defect centres can have one or more carbon atoms missing in the diamond lattice. They may even be replaced with a different element.
When diamonds have a sufficient quantity of the defect centres, they can absorb light differently, resulting in a visible colour difference.
“Different defects produce different colours,” said Byrne in the interview. “Sufficient nitrogen atoms in a diamond will make a diamond appear yellow; enough boron impurities can make a diamond appear blue.”
And while most other colours found in natural diamonds can be explained, the defect centres that cause the pink colour in diamonds remain unknown.
The researchers said the Argyle diamonds were the best samples to use because their intense pink colouring made changes in colour easier to monitor.