Diamonds in the Rough

by Zoe Smallwood, PhD student. Originally published in issue nine of Resonance.

Aurora Pyramid of HopeOften found glistening in tiaras and engagement rings, diamonds are considered a true symbol of wealth and opulence. Their popularity with jewellers is attributed mostly to their natural sparkle and colour (or, to be more precise, the lack of colour (completely colourless diamonds are extremely valuable). However, the smaller stones that don’t find their way onto rings and pendants are used in performance cutting tools and high pressure cells.

Whilst these two applications may seem completely unconnected (after all, why would a drilling tool need to be sparkly?), it is the extreme hardness of the stone that links the two. The word ‘diamond’ is derived from the Greek word 'adamas' which literally translates as ‘unbreakable’, with a hardness and stability greater than any other natural compound. This supreme strength is due to an "infinite" network of strong carbon-carbon bonds arranged in a tetrahedral structure.

Whilst the demands of drilling are much more intense than normal life, the extreme hardness and scratch resistance means that diamonds keep their sparkle and shine almost indefinitely; a hugely desirable feature in jewellery. Softer minerals such as opals, are brightly coloured but are less common in rings, particularly because they are more easily damaged.

Although they often appear colourless to the naked eye, only the highest quality diamonds are devoid of colour. The presence of trace amounts of nitrogen replacing carbon atoms in the structure (known as inclusion), varies the colour from colourless to yellow or even brown. The distribution of the nitrogen is as important as the quantity; if spread throughout the stone, the colour is much darker than if they are located in clusters at certain sites - the type of nitrogen inclusion can be detected using electron paramagnetic resonance to help validate the colour. Much rarer colours are caused by other imperfections such as boron (blue), graphite (black) or defects in crystal growth (red). The Aurora Pyramid of Hope, currently an exhibit in the Natural History Museum, displays 296 natural diamonds, each a different colour!

Due to cost and ethical considerations, more and more consumers are buying synthetic gemstones instead of those mined from the ground. Synthetic diamonds can be formed at high pressures and temperatures by crystallising molten carbon in a molten metal solvent, using ‘seed diamonds’. However, the predominant use of these synthetic diamonds is currently in industrial settings. Synthetic diamonds are often more yellow than their natural counterparts due to the inclusion of more nitrogen from their formation process, limiting their aesthetic value in an industry which prizes colourless diamonds.

More recently, another form of synthetic diamond has begun to appear. Moissanite, a crystalline form of silicon carbide, has a hardness only marginally less than diamond and is normally used for cutting of precious metals in the jewel industry. Moissanite is named after Dr Henry Moissan, who discovered trace amounts of the stone in a meteor crater in the late 1800's. The sparse amounts formed naturally meant it was not until 100 years later that an industrial process for manufacturing jewelleryquality stones was developed.

Unlike diamonds, moissanite is only formed synthetically, making it a cheaper and less ethically concerning alternative to some mined diamonds. A bonus is that moissanite has a higher refractive index, making them look more 'sparkly'! So, next time you are admiring a particularly striking 'diamond' ring, there’s a high possibility the stone may be the same compound used to cut the metal it is held in!

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