What Gives a Diamond Its Sparkle?

Diamonds are a girl’s best friend and we do love to watch them shine and sparkle in the light. But what really makes diamonds sparkle?


How Does a Diamond Sparkle?

Natural diamonds taken from the ground are dull. They have no sparkle on their own. The sparkle comes from the way the jeweller cuts the stone. And the young physicist Marcel Tolkowsky perfected this craft in 1919.

While Tolkowsky’s family were all jewellers, Marcel wanted something different for his life. He went to the London University and received a Ph.D. in engineering. Still, his background in diamonds made him curious as to how one could make diamonds sparkle even more.

It was Tolkowsky who discovered how to cut the 57-facet, or brilliant cut, diamond shape that nearly all diamonds use today.

What happens is that the cut creates a kind of kaleidoscope which makes a lot of pathways for light to follow. The reason a diamond seems to sparkle is that the light that enters the gem has to bounce around all these pathways. That gives the illusion that the diamond is sparkling.

There are several key aspects to a diamond’s shine & clarity.

Brilliance is the measure of the white light that emanates from the facets of the diamond. Picture this, if a diamond is lit from directly above, the light that enters the diamond will strike one lower facet, and then bounce to a second at a near-perfect 45-degree angle. This angle will reflect the light back out of the gem, which is how we see the sparkle. Tolkowsky proved that if the diamond’s cut is too steep, the light will reflect out of the side of the stone; and if the cut is too shallow, it will leave through the bottom of the stone.

Fire is the effect called chromatic dispersion. That means that when light strikes a diamond, it gets refracted as it enters the gem. Because white light is actually a mixture of all the colors of the rainbow, when it is refracted you can see some of the colors separated from the white. That is why you sometimes see rainbows of color in a diamond. But the fire is also the result of the diamond’s symmetry and proportions. A few factors that result in a diamond’s fire are star facet length, lower girdle length, pavilion angle, facet junctions, what angle a light enters the diamond, and where the light exits the diamond. Fire is best observed in dim conditions, like darkly lit restaurants.

One last element of a diamond’s sparkle is the dark side of the diamond or the contrast. Not every facet of a diamond will light up at once, which means that any dark objects around the diamond will be reflected on the gem’s surface.

The reason the dark side is so important is that our visual cortex reacts strongly to high-intensity contrast edges. When there is a close proximity of brightly lit and dark facets, our brains enhance the perceived brilliance of the diamond, thus making it seem like the diamond is flashing in the light.

Scintillation is another important aspect of the diamond. It is the intense black and white sparkles you see when a diamond moves. Usually, you’ll see this in well-lit areas, where a diamond’s fire is nearly completely absent.

Polish is the final ingredient to the sparkle of a diamond. Well-polished facets will help reflect the light and cause the diamond to sparkle more, while a poorly polished diamond will reduce the intensity of the light refracted in and out of the diamond, thus causing less sparkle.

So, you see that a lot of work goes into creating the beautiful gems that we see and love today. Detailed science and artistry go into each diamond, which makes it worth the money we spend on them. And now you know why diamonds sparkle!

Watch Neil deGrasse Tyson’s explanation on why diamonds sparkle.

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