Why do diamonds sparkle?

Before World War II only 10% of engagement rings had diamonds. But in 1947 De Beers introduced the new tag line to its engagement ring ads. It struck the perfect emotional chord among young lovers. By 1990 80% of engagement rings would have a diamond sparkling at their centre.

But what gives a diamond this seductive sparkle? Why is it so?

Actually a natural diamond is a rather dull affair. The sparkle comes from the way the stone is cut by the jeweller. But it was a young physicist named Marcel Tolkowsky who perfected this arcane art in 1919.

Tolkowsky’s family were jewellers. They’d already been in the diamond business for nearly a century when he was born. Tolkowsky took a different path, leaving the family home in Antwerp, Belgium, for London University and a PhD in engineering. But the apple did not fall too far from the tree. Tolkowsky found himself drawn to the study of diamond optics with the aim of maximising sparkle. He discovered the 57-facet “brilliant cut” – the quintessential shape in which almost all diamonds are cut today.

Tolkowsky’s cut creates, in effect, a very expensive kaleidoscope. The 57 facets create a huge number of internal pathways along which light entering the stone can travel. The light that appears to pour from a diamond under certain lighting is the result of Tolkowsky’s pattern bouncing light around. His work maximises two key aspects of a diamond’s shine – brilliance and fire.

Brilliance is a measure of the white light that beams from some facets of a diamond. When lit directly from above, light entering the diamond strikes one lower facet, and then a second, at near-perfect 45-degree angles (see illustration). This reflects the light back out the top of the stone, so that to the viewer it seems light is streaming from inside the diamond. As Tolkowsky showed, if the diamond’s cut is angled too steeply the light is reflected out the side of the stone. Too shallow, and it exits out the bottom.

The second illustration shows a different light pathway. Light striking the diamond at an angle gets refracted, or bent, as it enters the stone. As Newton discovered in his experiments with prisms, white light is a mixture of all the colours in the rainbow – and that by refracting it these component coloours can be separated out. This effect, which generates the little rainbows of colour that diamons create, is called chromatic dispersion – or in gemmological terms, “fire”.

But there’s one final element to the diamond’s sparkle – and it relates not to light, but to dark. Unless the diamond is very evenly illuminated from all sides not every facet will be lit up. Dark objects in the stone’s surroundings will also be reflected in the stone’s surface, making some facets appear dark. Your pupil, for example, is a very dark object, so when you look closely at the dark areas in a properly cut diamond you are seeing into the depths of your own eye!

Why is the diamond’s dark side so important for its sparkle? The most compelling reason our eye is caught by the sparkle of a diamond resides in the brain. Parts of the visual cortex respond strongly to high intensity contrast edges, and so the close proximity between brightly lit and very dark facets enhances the perceived brilliance of a diamond. The visual impact is enhanced by any slight movement of the stone, which causes a dramatic flashing and different facet areas to light up and then dim.

Other areas of the visual cortex are set off by sudden changes in brightness. These areas alert the frontal regions of the brain to get our attention, which is why diamond sparkle is so arresting.  And once captivated, the emotional connotation areas of our brains may kick in with the symbolism and associations of the stone.Is a diamond forever? De Beers’ line is not strictly true. Once a diamond leaves its dark, pressurised underground womb, it is subject to the forces of change. The carbon atoms are inclined to rearrange themselves into the lower density form – graphite (black pencil lead). And recent findings show high intensity light can eject carbon atoms from the surface – given enough time a diamond could evaporate! But these processes are very slow. On human timescales the brilliance of diamonds remains undimmed through lifetimes of wear.

Related reading: Diamonds not behaving like diamonds

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