Research: Diamonds are…
…A Girl’s Best Friend?
Ombré diamond engagement ring photographed by Simon Martner
My second post in a series of 'research' blog posts where I shall share what I learn as I go long. Currently I am looking into diamonds and how they have earned such a special position in our society.
Diamonds have become so elevated in Western culture that they have become almost wholly removed from the earth and now belong to the world of semiotics or, for the cynical, the century's most successful marketing campaigns. So what exactly are diamonds and what makes them special?
Roland Barthes, a French philosopher writing in the 1960s, summarised the mysterious qualities of the diamond as a song of ice and fire (long before George RR Martin's fantasy!)
“And yet, it seduces; hard and limpid, the diamond has a third symbolic quality: it glistens. Here it is incorporated into a new magical and poetic domain, that of the paradoxical substance, both lit up and stone cold: it is nothing but fire and yet nothing but ice.” 
But what exactly are they?
It is hard to get down to crystals, elements and atoms without getting very technical and potentially very dull if you are not really into that sort of thing. I am going to try and keep this short and sweet and avoid going too heavy with the diagrams.
Diamonds are carbon, just like coal, graphite, and about 18% of the mass of the human body.  So why are we not all glittering in the sun like Edward from Twilight? That would be due to the crystal structure.
A crystal is made of elements or compounds whose atoms have a repeating arrangement. They have distinct units which stack up over and over a bit like the puzzle pieces in this tessellation. Graphite is an example of a form of carbon that looks nothing like a diamond.
Simple puzzle tessellation.
Diamonds and graphite are both made of the same single element, carbon. The outer layer of a carbon atom has four areas where it can bond with other atoms, like the tabs and slots of the puzzle piece. In graphite the atoms link up to form flat layers that stack on top each other. In diamonds each carbon atom is linked up with its neighbours in a tetrahedral-shape so that the layers are joined.
I imagine this would look like a mille-feuille (representing graphite) vs a croissant (representing diamond). You can just tell that the mille-feuille is going to fall apart as soon as you bite into it because the layers of pastry are not attached to each other except by cream and strawberries. Meanwhile the croissant has an internal structure which holds together. (Though obviously you can bite through this structure, it is just a croissant after all.)
Mille-feuille and croissant. Open source.
How hard are they?
Diamond is the hardest known substance. Even its name comes from the Roman adamas/ Ancient Greek adámas, “unconquerable, invincible” like the mythical metal adamantine. In 1822 an Austrian mineralogist called Friedrich Mohs developed a scale of hardness based on what could scratch what with diamond at the top of the scale at number 10 and corundum (rubies and sapphires) at number 9 though the scale is not really linear and diamonds are actually significantly harder. This does not mean they are as invincible as the name would imply. There are four directions in the structure of a diamond that are easier for it to be split along, a bit like cutting with the grain in wood. This is known as cleavage but the average diamond ring is unlikely to be struck such an angle that it would be caused to split.
A Descriptive Catalogue of Diamonds in the Cabinet of Sir Abraham Hume, Bart. ... By the Count de Bournon. (Letter from the Count de Bournon to Sir Abraham Hume on the crystallization of the diamond.) With plates. Translated from the French [by Sir A. Hume]. Source: British Library
Why are they so sparkly?
As well as being exceptionally hard, diamonds are also very dense. (3.51g per cubic centimetre.) Carbon is not usually a dense element but the carbon atoms in a diamond are tightly packed together. The density and the structure means that it slows light down to nearly half the speed that it would travel in space. Within the crystal, light is bent (refracted) and reflected. The way a clear object appears in air, for example a glass door is not usually considered completely invisible, is called lustre. Diamonds set the standard for bending light and are described as having an adamantine lustre. Flat surfaces, facets, are cut into diamonds to control how the light enters the crystal and give them the famous lustre and sparkle as the light bounces around within the gem before reflecting back to your eyes.
Diamonds are also very transparent. A glass door will not block the majority of visible light but will block other parts of the spectrum. Whereas diamonds allow almost the whole spectrum to pass through, from microwaves to infrared.
A lot of people do not realise how many uses diamonds have outside of the jewellery industry. Many of diamonds' properties make them useful for scientific and industrial use. One property is that it is a great thermal conductor. This is one of the things that give diamonds their iciness. If you touch a diamond to your lip (presuming the diamond is room temperature and your lips are normal body temperature) the diamond will draw heat from your lip making it feel cold - icy! 
That's all for now!
- ROLAND BARTHES: From Gemstones to Jewellery was first published in Jardin des Arts, 77 (April), 1961; Oeuvres complétes vol. 1, 911-14. available online here.
- The Nature of Diamonds edited by George E.Harlow - this is my main source of information for this post and I would highly recommending it for learning about all facets (sorry, not sorry) of this precious stone.