Why are Pink Diamonds Pink?
GIA Researchers Dive Deep into their Crystal Structure
October 18, 2019
Natural pink diamonds are among the most valuable and rare of Earth’s treasures. Top, vivid-coloured stones can bring in more than $2 million (£1.5 million) per carat at major auctions. Such prices come from their rarity as much as their beauty – only a tiny percentage of diamonds have pink colour, and only a tiny percentage of these have a rich, vivid colour.
Pink diamonds created in a laboratory, however, are quite different from most of their natural counterparts. Manufacturers can’t replicate the way the vast majority of these fancy coloured diamonds formed in nature, according to GIA researchers.
Employing GIA’s immense database of more than 90,000 natural pink and related coloured diamonds graded between 2008 and 2016, GIA researchers Dr Sally Eaton-Magaña, senior research scientist; Troy Ardon, research associate; Dr Karen V. Smit, research scientist; Dr Christopher M. Breeding, senior research scientist, and Dr James Shigley, distinguished research fellow, produced the most detailed and comprehensive gemmological analysis of pink diamonds to date, published in the Winter 2018 edition of the Institute’s quarterly journal, Gems & Gemology.
The sample set of 90,000-plus diamonds includes all those submitted to GIA from 2008 to 2016 with pink as a primary colour, and also spans the colour hue range from red to purple and saturation range from faint to dark, along with brown diamonds, which share a similar cause of colour as pink and related stones. Many of those 90,000 diamonds were small, with low colour saturation.
This research confirms that the colour of 99.5% of pink diamonds comes from distortion in their crystal structure, not from trace elements, such as nitrogen, which causes yellow colour in diamonds or boron, which causes blue. In pink diamonds containing nitrogen, the colour is generally concentrated within parallel narrow bands called glide planes, lamellae or pink/brown graining, depending upon the colour. These lines are visible under a microscope and cutters orient them perpendicular to the table to maximise bodycolour.
While many colour-producing defects can be introduced through laboratory treatment processes, atomic level distortions in a diamond’s crystal structure created by plastic deformation cannot.
“We know that plastic deformation is associated with the vast majority of diamonds with these colours, but we still do not know the actual atomic structure of the defect causing the colour,” says Breeding, who notes that the pink colour is caused by a broad absorption band centred at 550 nanometres (nm) in spectroscopic analysis. Spectroscopic analysis is an important gemmological tool to measure the impurities and other defects within a diamond (and other gemstones) that give rise to specific absorption peaks or bands in the visible spectrum. The researchers said that there is no known method of replicating plastic deformation with the 550 nm absorption band by a laboratory treatment or growth process.
All fancy coloured diamonds submitted to GIA’s grading laboratories are subjected to rigorous analysis to be certain that the colours ‒ and the diamonds themselves ‒ are of natural origin, which makes the GIA database extensively detailed and valuable for research, Shigley said. He added that the team selected a representative sample of 1,000 pink diamonds to examine even more thoroughly for the Gems & Gemology article.
How are Laboratory-Grown Pink Diamonds Created?
There are laboratory-grown pink diamonds on the market, but they are created by three different methods, said Eaton-Magaña.
“The first, and by far the most common, is through irradiating (exposing to radiation) a laboratory-grown diamond that was grown with nitrogen impurities, then subjecting it to moderate temperatures (600°C to 1000°C) afterwards,” she said. “The vast majority of pink and related colours are made using this method.”
This process creates a defect in the crystal lattice (caused by a missing carbon atom adjacent to a nitrogen atom within the lattice) called a nitrogen-vacancy centre that is responsible for the colour. Eaton-Magaña said a tiny percentage of natural pink diamonds have this nitrogen-vacancy centre, which is identifiable by spectroscopic lines at 575 nm and 637 nm.
“These nitrogen-vacancy pinks comprise about one half of one per cent of the natural pinks in the GIA database,” she said. They are type IIa diamonds (a rare type of diamond with an exceptionally pure chemical composition: almost all carbon, with negligible amounts of nitrogen or boron) with a very uniform pink colour and no visible coloured lamellae. These stones are sometimes termed Golconda pinks, but they don’t necessarily have a connection to the Indian mine that closed in the late 18th century.
Generally, she noted, natural nitrogen-vacancy pinks have pale colours, while the laboratory-grown and treated diamonds are more highly coloured. Additional testing, such as at a gemmological laboratory, is essential to determine the cause of colour in such diamonds.
The second method cited by the researchers occurs only in chemical vapour deposition (CVD) laboratory-grown diamonds, where a 520 nm spectral band is created during growth that results in an orangey-pink colour. GIA has only seen a few of these stones, they say, and the colour was not as vibrant as other pink lab-grown diamonds.
The third method, which is even rarer, Eaton-Magaña said, is achieved by adding high amounts of silicon to the CVD growth process which, when exposed to UV illumination, creates reversible colour change between the stable pink colour and a temporary blue colour.
Size and Colour Statistics: Are Pink Diamonds Rare?
Of the group of diamonds in this study 47% were unmodified (no other colour observed) pink; 28% were purplish pink to pinkish purple; 17% were brownish pink to pinkish brown; 10% were brownish orangey-pink to orangey-pink; 3% were brown; 1% were purple-brown, purple-grey and purple; and 0.9% were red, brown-red or orange red. Of the unmodified pink diamonds, 54% were graded faint to light pink.
While brown diamonds are among the most common fancy colours in nature, the low percentage of them in this group suggests that most are sold into the market without grading reports. The brown colour is accompanied by other hues, such as pink or yellow, or the brown stones are treated to produce other colours.
The vast majority of the group (83%) weighed less than one carat, with 56% of them being under one-half carat. The dominant shapes were round (24%), pear-shaped (20%), rectangular (16%) and cushion-cut (13%).
The research ‒ the most extensive ever carried out on pink and associated coloured diamonds ‒ found that 25% are type IIa, which is much higher than with colourless to near-colourless diamonds (where the percentage is <5%), Breeding said.
The majority of type Ia pink diamonds (diamond containing clusters or aggregates of nitrogen atoms as impurities in the crystal lattice) almost exclusively come from two sources: Australia’s Argyle mine and Russia, which also happen to be the most prolific and consistent producers of such stones. Type IIa pink and other type Ia pink diamonds come from other sources – such as Tanzania, South Africa and Brazil – but there is no reported regular production from any mine.
“It’s unusual that these types are from very specific locales, especially because they are so rarely found in other sources,” Breeding noted, adding that because Argyle is the dominant source for highly saturated (coloured) pink, purplish-pink and red diamonds, the decline in the numbers of such stones will be substantial after the mine closes next year.
Research Aids Automated Detection of Treatment/Lab-Grown Diamonds
This pink diamond study enables GIA to better understand how these rare and beautiful diamonds formed and the properties that give them their colour. The researchers found many new details at the atomic level by analysing the absorption spectroscopy and photoluminescence data. These findings add to the understanding of these amazing stones – which can enable faster, more accurate identification.
The research from this study reinforces GIA’s development of an upgrade for the GIA iD100® gem-testing device that adds a function to accurately screen pink and related colour diamonds for treatments and synthetics.
Russell Shor is senior industry analyst at GIA in Carlsbad.