Diamond Sparkle

Diamond bends light more than almost any material used in jewellery. Light slows and changes direction on the way in, bounces around inside, and leaves again through the top. What you see as sparkle is that journey, and the journey is controlled entirely by the angles of the pavilion and crown.

Understanding the three components is genuinely useful when comparing two stones, because a diamond can be strong in one and weak in another. People often describe a stone as lacking sparkle when what they actually mean is that it has plenty of brilliance and almost no fire.

Brilliance is the bright white light that comes straight back at you. Light enters through the table, strikes one pavilion wall, reflects across to the other, and exits through the top.

That double bounce only happens inside a narrow band of pavilion angles. Too shallow and light passes out through the bottom of the stone. Too deep and it leaks out of the side. Either way you get a stone that looks dull or glassy in the middle, and no amount of clarity or colour will rescue it.

Brilliance is what a round brilliant maximises better than any other shape.

Fire is the flash of red, orange, blue and green you catch out of the corner of your eye. It happens because different wavelengths of light bend by slightly different amounts as they pass through the stone, so white light fans out into its component colours, the same effect as a prism.

Fire comes mostly from the crown. A taller crown with steeper angles produces more of it, which is why antique cuts such as the old European, and modern shapes with tall crowns like the Asscher, can look more colourful than a flatter modern stone even though they return less white light.

Lighting matters more for fire than for anything else. Fire shows under small bright point sources, candlelight, spotlights, sunshine. Under a flat office ceiling panel almost any diamond will look lifeless, which is worth remembering before you judge a stone in a shop.

Scintillation is the twinkle: the pattern of light and dark flashing on and off as the stone moves. A photograph cannot show it, which is why diamonds always look better in person than online.

It comes from the smaller facets, the upper and lower girdle facets in particular, each catching light for a moment and losing it again. A stone with many small facets produces fast, fine, glittery scintillation. One with fewer, larger facets produces slower, bolder flashes. Neither is better; it is the difference between a crushed ice cushion and a chunky one, and it is entirely a matter of taste.

1. Cut. By a wide margin the most important. Proportions decide whether light comes back at all. Every Ronora diamond is Excellent cut for exactly this reason.
2. Symmetry. Facets that do not line up send light off in inconsistent directions, which blurs the flash pattern and softens scintillation.
3. Polish. Microscopic surface marks scatter light at the point of entry and exit. Graded separately on the report, and Excellent is what you want.
4. Shape. Brilliant cuts sparkle; step cuts flash. An emerald cut is not a badly performing diamond, it is a different intention.
5. Cleanliness. Not a grading factor and the most common real cause of a dull diamond. A film of hand cream and soap on the pavilion blocks light before it ever gets in.
6. Clarity and colour. Last, and much further down than people expect. Only large or dense inclusions interfere with light return.

• Most brilliance: round, then princess and radiant. All three are cut specifically to return white light.
• Most fire for its size: radiant and cushion, both of which trade a little brightness for colour.
• Finest, glitteriest scintillation: crushed ice cushions and elongated cushions.
• Broad, slow flashes: emerald and Asscher. Step cuts are quiet by design.
• Watch the centre: every elongated brilliant, including oval, pear and marquise, carries a bow-tie, a dark band across the middle where your own shadow blocks the light. A faint one is normal. A strong one dominates the stone.

• Move it. Hold the stone still and you see almost nothing of what makes a diamond a diamond. Tilt it, rock it, walk with it.
• Change the light. Look under a spotlight for fire, by a window for brilliance, and in ordinary mixed room light for how it will actually live.
• Look for dark patches. Persistent dead zones that stay dark as the stone moves mean light is leaking, not reflecting.
• Step back. Sparkle is judged at arm's length, not through a loupe. Under magnification every diamond looks busy.
• Clean it first. Warm water, a drop of washing up liquid, a soft brush on the back. A dirty stone can look a full cut grade worse than it is.