What Gives Diamonds Their Sparkle?

Diamonds are prized for several reasons - they are very hard, making them an important component in cutting technology - and they are intrinsically beautiful. Diamonds sparkle and shine like no other gemstone, which is why they are so prized by bespoke jewellery  makers. Manufacturers of fake diamonds try to recreate this characteristic and some come close, but never with complete success. So what makes diamonds sparkle?

The science of the sparkle

It turns out that the diamond's sparkle, and therefore beauty, is directly linked to the other property, its hardness. The key phrase here is ‘total internal reflection'. Simply put, it's the way that diamonds manage to bend light within themselves to an extent that other substances do not.

Diamonds' hardness and stability are a result of their structure. Graphite and diamond are both forms of carbon - as is the sooty smoke that rises from a candle. But whereas the graphite in a pencil is soft - otherwise it wouldn't be a very good pencil - diamond is so tough that nothing rubs off. That's not to say it's indestructible: diamonds will burn away to carbon dioxide at high enough temperatures, leaving nothing left. Graphite is effectively made from sheets of carbon atoms, which slide over each other to be left on the sheet of paper in the case of a pencil. But in diamonds, the carbon atoms are all rigidly cross-linked, making for an extremely stable structure.

Shedding light on the process

Why does this make diamonds sparkle? Well, if you've ever wondered where a rainbow comes from, you've got your answer. Light travels at 300,000 km per second in a vacuum. Through air, it's a little slower, and through water and glass, slower still.

Imagine a horizontal line of soldiers marching into the sea. If they march straight into the waves, they will all slow down at the same time. But if they hit the sea at an angle, with one end of the line reaching it first, the line will become bent as some soldiers slow down before others. The same is true of light hitting a droplet of rain at an angle. The light is refracted - bent - which has the effect of scattering its different wavelengths into a spectrum (like heavy soldiers struggling in the waves more than their lighter comrades...). At the correct angle, the light is bent more than once and reflected back off the inside of the droplet (total internal reflection), back towards the source - which is why you only ever see a rainbow with your back to the sun.

Now, imagine the soldiers were marching into something with more resistance, like golden syrup... the line bends even more. Diamond's hardness bends light more than any other substance, so the effect is more pronounced. Light is scattered more, and total internal reflection occurs more frequently. Add to that some careful cutting, which accentuates the natural glitter, and you have a sparkle of unrivaled brilliance.