What for were the diamonds made coloured

Rough and Polished

An unusual experiment on precious gems made at the Moscow Institute of Steel and Alloys.
Few of the rank and file people can afford buying a coloured natural polished diamond, writes Natalia Vedeneyeva on www.mk.ru. Such ‘sparklers’ are very expensive. What the majority of people having mid-level earnings can afford are colourless diamonds at the price of RUB 7,000 – 8,000 for 0.1 carat. The researchers of the Moscow Institute of Steel and Alloys made up their mind to change the situation for the better and have developed a unique process of diamond colouring to change the colour of natural diamonds. Their price is about 30 per cent higher than that of colourless ones, but it is not 10 times higher than the price of natural coloured diamonds. The MK Weekly reporter visited the superhard materials laboratory and found out how to colour a polished diamond so that no one could distinguish it from a natural coloured one.

Coloured diamonds are so rare in nature that recently a small 2 ct blue polished diamond has gone under the hammer for about $10 million. However, at the Moscow Institute of Steel and Alloys it takes three days to duplicate the work of nature that takes millions years. How does it come?

It turns out that the ‘colourants’ are there inside every – even colourless – diamond, and these are nitrogen atoms. Everything depends upon their concentration and arrangement. Here, in the laboratory they make them move to those parts of a diamond crystal lattice where required.

However, according to Nikolai Polushin, Chief of the Laboratory, you cannot change the colour of each and every natural diamond, but only of those with a very low nitrogen concentration – below 100 atoms per one million of carbon atoms (there is less than 3 per cent of such rough diamonds in nature). That is why a natural diamond is studied first with a special-purpose spectrometer. Then, an appropriate specimen is exposed to high temperature, high pressure (to achieve the necessary nitrogen atoms re-arrangement in the crystal lattice) and irradiation in the cathode-ray accelerator (to obtain vacant sites in a diamond crystal lattice called ‘vacancies’).

In fact, the crystal colouring depends upon certain defects in a diamond crystal lattice, says Polushin.

To make a transparent stone a coloured one, it is necessary to displace one carbon atom from its crystal lattice using a terrestrial or space radiation so that a vacant site (vacancy) could form. Then, a single nitrogen atom should take place next to it. We perform all these processes in our accelerator, press and vacuum furnace. All efforts are aimed at creating certain colouring centres in the diamond lattice thanks to such a double defect. The centres should be uniformly arranged. The more such centres there are, the more saturated the colour is.

However, this is the way used to obtain pink diamonds only. The blue ones are obtained by another technique: by ‘colour implantation’ into a diamond.

– In nature, such crystals contain small boron concentrations” says Polushin. You can follow this way, but we took some other way: we implant hydrogen atoms into the diamond crystal lattice – and these atoms also make a diamond blue.
– How do you implant them?

– This takes place in the high-pressure unit, in the press. We put a diamond into it onto a special blend that decomposes at high pressure and high temperature with atomic hydrogen evolution. Then, at high pressure and high temperature these hydrogen atoms penetrate into the diamond crystal lattice.

Pink and blue diamonds prevailed among the artificially coloured diamonds that were shown to us by Nikolai Polushin.

– And what about yellow diamond?

– There are much more yellow diamonds in nature and they are cheaper than the colourless ones, that is why there is no sense in making diamonds yellow, although it is possible.

– And how are the crystals coloured black?

– The principle is even simpler: we achieve a high concentration of vacancies – vacant lattice sites – and then we carry out the heating process in such a way that the conversion of a diamond into graphite could occur in these sites. There appear many sites with graphite inside. That is why it seems that all the crystal looks black. I want to warn those who like such diamonds: sky-high prices for black diamonds in the stores are not justified. Few people know that more often than not, low-quality rough diamonds are used to produce black polished diamonds, for example, colourless crystal with impurities. The black colour just hides this defect, although it looks very nice.

– Do high quality natural black diamonds exist?

– They do, but they are rare.

– Who supplies rough diamonds to you?

– So far, all the roughs have come from Belgium. We have already sent a batch of lab-coloured crystals to the suppliers for a marketing research. We would like to cooperate with the Russian diamond miners, but it doesn’t work out. They refer to an intercorporate agreement under which they are committed not to finance the change of the colour of natural diamonds and not to perform this change. From the miners’ point of view, probably, it is correct – natural coloured diamonds are very rare and very highly priced.

– So, your technique could significantly run down the prices, could not it?

– No, it’s all fair and above-board, we state that our natural diamonds are artificially coloured. This is like comparing natural diamonds and synthetic diamonds.

– Is it possible to distinguish the artificially coloured diamonds without special instruments?

– No, it is possible to do with the use of special instruments only. But these analyses are ambiguous. We gave several crystals out of our batch to the gemological centre, Antwerp, and the experts could not tell our diamonds from natural coloured high-end polished diamonds.

By the way, this year the Moscow Institute of Steel and Alloys has become the first Russian university to offer the full-fledge education of bachelors of Nanomaterials Science. And it is quite possible that the graduates from this department will improve the diamond colouring techniques in future.

Source Rough&Polished