Icy white. Transparent as water. Harder than nails. Billions of years old. Diamonds are the most sought after gems on earth. But what really makes a diamond special is the way it handles light. That’s what makes diamond the king of bling. Nothing is as brilliant as a diamond. So what makes a diamond sparkle?

When a diamond is mined, nature has already determined color, clarity, and most of carat weight. But a rough diamond just looks like a transparent rock. It doesn’t sparkle. It doesn’t flash in rainbows. It doesn’t dance with light.

Cut is what makes a diamond perform in the spotlight. And this doesn’t just happen, it’s the result of the skilled craftsmanship of the diamond cutter.

It took thousands of years for our ancestors to learn how to use diamond to polish diamond. When modern diamond cutting was invented, man unleashed the brilliance that comes from diamond’s unique optical characteristics.

Today, the diamond cutting process often starts with lasers to cut the rough into chunks instead of the traditional saw. But after that, diamond cutting takes as much patience as ever. The cutter uses a wheel covered in diamond dust to slowly grind and polish each facet. It takes time and skill to do it right.

To create the sparkle of the round brilliant diamond, diamond cutters add 58 precisely polished facets to capture light. The angles, proportions, and alignment of the facets direct the light inside the diamond and create brilliance. Diamond facets are like a hall of mirrors that reflect light from all angles and send it back to your eyes.

The cutter makes that happen by getting the angles just right. Too shallow and the light just passes through. Too deep and it leaks through the side instead of hitting your eye with sparkle. Either gives you dark spots in your diamond. That’s why a poorly cut diamond looks like glass.

A beautifully cut sparkling diamond shows three special optical effects brilliance, fire and scintillation.

Brilliance describes diamond brightness, the reflections of white light that make diamonds look like they are lit from within.

Fire describes the rainbow flashes of diamond dispersion: light bent into spectral colors. In bright sunlight a diamond will reflect rainbows around you like a prism.

Scintillation is the spots of light that flash as the diamond moves and the pattern and contrast of bright and dark areas in a diamond. Scintillation changes as the diamond moves: it’s the sparkle of the diamond.

Cut unleashes the potential of diamond sparkle. When a beam of light hits a diamond, it slows down to just 77,000 miles per second. That’s more than 100,000 miles per second slower than in air. That speed difference bends light into rainbows of color and flashes of brilliance.

Diamonds sparkle differently in sunlight versus candlelight or daylight versus office lights because they reflect the environment around them. When you look at your diamond, you are also seeing a reflection of everything around you, including yourself.

The dark parts of the pattern you see in a diamond are a reflection of your face. You can test this yourself. Hold a diamond at arm’s length and look at its brightness and its pattern of dark and light. Now, gradually bring it closer to your eyes. See how the area of dark in the diamond’s pattern grows?

That’s a reflection of you in the sparkle of your diamond, a diamond sparkle selfie. Every time you look at your diamond, you are looking in a tiny sparkling mirror.

Reflection, refraction, brilliance, fire, sparkle. Diamonds are meant for the spotlight. That’s what makes them the world’s most coveted gem.