The short answer
A well-cut round brilliant. Nothing else on the market sends as much light back at the person looking at the ring, and nothing has come close in a century of trying. If you want an answer with no qualifications attached, that is the answer.The qualification is the part worth your time. Shape moves sparkle a little. Cutting moves it a lot. Two round brilliants of the same weight, colour and clarity can sit side by side on a tray and one of them will look flat, because a cutter kept weight the stone should have lost. Meanwhile a carefully cut oval will out-sparkle a lazily cut round every time, and it will cost less per carat.
If you take one thing from this pagePick the shape you like, then spend your effort on the cut of the individual stone. Choosing a round and then buying a badly cut one is the most common way people end up disappointed by a diamond they paid a premium for.
Three things people call sparkle
Sparkle is not one property. It is three, and they behave differently enough that a stone can be strong in one and weak in another.
One round brilliant, drawn three waysA schematic of what each component looks like face up, not a photograph or a measurement. Brilliance, fire and scintillation are the names GIA uses for the appearance half of its cut grade, and the drawing is only here to show that they are separate things.
They trade off. Light can leave a facet as white or as colour, not both, so a cut tuned for maximum brightness gives up some fire, and the old cuts that throw the most colour are noticeably less bright. Facet size works the same way. Many small facets give you a fine constant glitter; a few large ones give you slower, bigger flashes that carry more colour. Neither is a better stone. They are different tastes, and it is worth working out which one you are describing when you say you want more sparkle. Our guide to the prettiest diamond cut lays out the four families of faceting side by side, if you want to see which flash pattern appeals.BrillianceWhite light sent straight back at you from most of the face at once. This is the part that reads as bright rather than busy, and it is what a good pavilion angle buys you.
FireThe share of that light which comes back split into colour, in thin blades of red, orange and blue at the edges of facets. Fewer, narrower and much easier to lose than brilliance.
ScintillationWhich facets are lit changes every time the hand tilts, so the flashes switch on and off. How many, how big and how fast is the difference between a fine glitter and a slow blink.
Why the round brilliant wins
A round brilliant has 57 facets, or 58 when the cutter leaves a tiny flat facet at the point underneath. What matters is not the count. It is that every one of them is tilted out of the plane of the table at an angle chosen so light entering the top comes back out of the top. Marcel Tolkowsky published the proportions in 1919 and modern cutting is still a refinement of them.A circle helps too. Light arrives at a round stone from every direction equally, and no part of the outline is further from the centre than any other, so there is no corner or point where the facets run out of room. Every shape with a long axis or a sharp corner has somewhere the geometry has to compromise.There is a practical consequence that matters more than the physics. The round brilliant is the only shape GIA issues a cut grade for, because it is the only one cut to proportions consistent enough to score. Seven components go into it, and the overall grade is the lowest of the seven rather than an average.
One round brilliant, scored on seven components
For every other shape the cut line on the report is blank, or carries only polish and symmetry, which tell you how neatly the facets were finished and nothing about whether the stone returns light. That is the real argument for a round if sparkle is your priority: not that a round is guaranteed to be bright, but that it is the one shape where somebody independent has already checked.Appearance
BrightnessExcellent
FireExcellent
ScintillationExcellent
Design
Weight ratioVery Good
DurabilityExcellent
Craftsmanship
PolishExcellent
SymmetryExcellent
Cut grade on the reportVery Good
The system is deductive: the overall grade is the lowest of the seven, never an average of them. Component names and the rule are GIA's, published with the cut grading system it launched in January 2006. The grades shown are an illustration, not a real stone.Where the other shapes land
Here is the whole set on one scale, brightest first, with each outline drawn at the size it would be at one carat.
Loose stones on a bench under ordinary ceiling light. The gap between the liveliest and the dullest here has nothing to do with which shape they are.
Eight shapes ranked by how much light they send backWell-cut 1.00 ct stones, drawn to one scale, longest axis verticalThe ranking is our editorial ordering of how these shapes behave, based on the faceting each one uses and on the stones we handle. It is not a measurement, and nobody publishes a percentage of light return you can trust across shapes. Treat the order as reliable and the gaps between neighbours as approximate. Millimetre figures are the published averages for a well-cut 1.00 ct stone in each shape.
Two things in that list surprise people. The first is how far down the princess sits. It was the second most popular shape on the market for years and it is genuinely bright through the middle, but the square outline leaves the corners with less to work with and the cut carries a lot of hidden depth. The second is how well the crushed-ice cuts do. A radiant or an elongated cushion returns nearly as much light as a round; it just arrives as a fine dense shimmer rather than as separate flashes, so some people read it as less sparkly when it is simply busier.The bow-tie is the risk that runs through the middle of this list. Any brilliant cut stretched along one axis can develop a dark bar across its waist, where the pavilion facets reflect the shadow of your own head back at you instead of light from the room. Ovals, pears and marquises all carry it. It never appears in a listing photograph, because listing photographs are taken in a light box lit from every side, and it is often the first thing you notice on a hand.Round brilliantFine, constant, everywhere
Fifty-seven facets worked out to point light back at whoever is looking at the ring. It is also the only shape with a cut grade on the report, so it is the one shape where you can check the claim rather than take it.
RadiantDense crushed ice
Brilliant facets inside a rectangle with clipped corners. The flashes are small and packed tight, which reads as a shimmer rather than a set of distinct sparkles. It is also the shape that hides a warm tint best.
Elongated cushionSoft crushed ice
The same crushed-ice behaviour with rounded corners and a slightly slower, warmer flash. Cushions vary more than any other shape from cutter to cutter, so this one rewards looking at the individual stone.
OvalBright, with a risk
A round brilliant stretched. It keeps most of the light return and picks up a new problem: a dark band across the middle called a bow-tie, which is invisible in a lightbox photograph and obvious the moment the stone tilts.
PearBright at the belly
Brilliant faceting again, with the bow-tie risk of the oval and a point that has to be set behind a claw. Well-cut examples are lively; the badly cut ones go dark across the widest part.
PrincessBright centre, quiet corners
Lively through the middle and noticeably calmer out at the corners, where a square outline gives the facets less to work with. Princesses also carry more of their weight below the girdle, so a heavier stone can look smaller.
EmeraldBroad, slow flashes
Long parallel steps instead of tilted triangles. It throws a few big flashes as the hand turns and then goes quiet, which is the effect it was designed for rather than a shortfall.
AsscherConcentric, hall of mirrors
A square step cut with deeper corners than an emerald, so the steps stack into a tunnel down the middle of the stone. Quietest of the lot, and the most orderly to look at.
An emerald cut is not a failed brilliant
Count the facets on an emerald cut and you get 57, the same as a round brilliant. Everything about how it behaves comes from where those facets point.
Two facet plans, seen from aboveFacet counts are the GIA figures for the standard versions of each cut. Both are 57, which is why counting facets tells you nothing about sparkle.
A brilliant is dozens of small mirrors aimed in dozens of directions, so something is always catching the light. A step cut is a handful of long mirrors lying parallel to the table, so the stone stays calm until one of them lines up with a lamp and then throws a single broad flash across the whole facet. That is the effect it was cut for. An emerald cut that flickered would be a badly made emerald cut.The trade-off is honesty. A step cut has nowhere to hide an inclusion or a warm tint, so it needs better clarity and colour than a brilliant of the same size to look right, and we set out what that costs in the emerald cut, honestly. If constant movement on the hand is what you are after, do not buy a step cut and hope. Buy a brilliant.Round brilliant57 facetsTriangles and kites, each one tilted out of the plane of the table. Light that goes in comes back at you from dozens of small mirrors pointing in dozens of directions.
Emerald cut57 facetsLong steps running parallel to the table and to each other. The same light comes back off a handful of big flat planes, so you get broad flashes instead of scintillation.
What actually kills sparkle
Almost every dull diamond we are asked about is dull for one of four reasons, and none of them is the shape on the report.The first is the pavilion, the cone of facets under the girdle. Light has to reflect off two of them to come back out of the top, and the angle they sit at decides whether it does. Cut the pavilion too shallow and the light passes straight out of the bottom. Cut it too deep and it slides out of the side. Both mistakes exist for the same reason: a deeper or shallower stone keeps more of the rough, and rough is what the cutter paid for.
Setting matters too. A deep bezel or a heavy gallery shades the pavilion, and a stone that cannot take light in from the side has less to give back.
One ray of light, three pavilionsA schematic of the ray paths, not a ray trace. The crown is identical in all three drawings and only the depth below the girdle changes. A round brilliant graded Excellent for cut usually sits around 59 to 62.5 per cent total depth with a table between about 53 and 58 per cent of its diameter, which is the pair of numbers worth asking for when a report has no cut grade on it.
The second is dirt, and it is by far the most common. A film of soap, sunscreen and hand cream collects on the underside of the stone, where you never look, and it absorbs the light before it can reflect back up.Cut too shallowA flat watery centre
The pavilion facets meet the light at too gentle an angle to turn it around, so it passes straight through and out of the bottom of the stone. You see the tablecloth through the middle of the diamond instead of the diamond.Cut for lightThe whole face lit
Light crosses the stone, bounces off both pavilion facets and leaves through the table on its way back to your eye. Every flash you see is a ray that made this trip.Cut too deepA dark hole in the middle
Extra depth is weight the cutter kept, and it costs you the second bounce. The ray goes too far down, meets the far facet at the wrong angle and slides out of the side instead of returning.One ray of light, two undersides
Warm water, a drop of dish soap and a soft toothbrush along the underside fixes it in two minutes. Most people who tell us their diamond has stopped sparkling have a clean stone within the hour, and our care guide covers what not to use.The third is a bow-tie in an elongated stone, which no amount of cleaning will move. The fourth is the room. A diamond has no light of its own; it can only return what falls on it. An office with flat overhead panels, an overcast afternoon indoors and a restaurant lit by one warm globe all produce very little for a stone to work with, and every diamond looks quiet in them. Showroom ceilings are hung with spotlights for precisely this reason, which is also why a stone that dazzled you at a counter can feel flat in your kitchen.Clean underneathLight crosses the stone, reflects off both pavilion facets and leaves through the table. That return is what you see as brilliance.
Two weeks of hand creamA film thinner than a fingerprint sits against the outside of the pavilion. The light goes in and does not come back. The stone reads grey.
Lab-grown, natural and moissanite
A lab-grown diamond and a mined diamond sparkle identically, and there is nothing hedged about that. Sparkle is geometry acting on a material, and both stones are carbon in the same crystal structure with the same refractive index of 2.42 and the same dispersion of 0.044. Put a well-cut lab-grown round next to a well-cut mined round and no gemmologist alive can tell you which is which by how it behaves in the light. They need a machine, and the machine is looking at trace growth features, not at brightness.Nor do lab-grown diamonds lose their sparkle over time. Neither kind does. They both dull because they get dirty, and they both come back.Moissanite is a genuinely different material, and the honest answer is that it does sparkle more. It bends light harder than diamond and splits it into colour more than twice as strongly.
What a material can do with light, before anyone cuts itRefractive index across, dispersion up, with the dashed lines drawn through diamond
Sitting up and to the right of diamond sounds like a win, and for a lot of people it is the exact thing they dislike. That much dispersion throws visible rainbows in ordinary daylight, which reads as a disco-ball flicker rather than as the white flash people associate with a diamond. It is also what gives moissanite away across a table. Some buyers love it. Plenty of others try one and find it looks synthetic for a reason they cannot name, and the reason is this graph. Our guide to diamond alternatives plots the rest of the lookalikes on the same two axes.Light returned as colour
White sapphire
Cubic zirconia
Diamond, lab or mined
Moissanite
Light returned as white
Refractive index and dispersion figures as published by GIA. Diamond is 2.42 and 0.044; moissanite is roughly 2.65 to 2.69 depending on direction, plotted here at 2.67, with dispersion of 0.104. These are properties of the material only. What any individual stone does depends on how it was cut.How to check a stone before you pay
None of the above helps unless you can apply it to the stone actually being offered to you. Three requests do most of the work, and any seller worth buying from will meet all three without being pushed.Ask for a video of that stone, not a stock image of the shape, in ordinary room light, moving. Not a light box, not a spotlit turntable. Watch for a dark band across the middle as it tilts, and for a centre that goes flat or black. Both are visible in ten seconds of honest footage and neither shows up in a photograph.Then ask for the cut grade. If it is a round, you want Excellent, and you want to see it on the report rather than in the listing copy. If it is any other shape there is no cut grade to have, so ask for the table and depth percentages instead and treat anything far outside the usual band as a stone cut to keep weight. Polish and symmetry should both be Excellent or Very Good whatever the shape.
The one question that sorts sellers quicklyAsk which stones in your budget they would not put in a ring, and why. Anyone who cuts for weight rather than light has a drawer full of heavy, dull stones at attractive prices, and a seller who will not name one is selling you the drawer.
For what it is worth, our own answer to the question in the title is a round brilliant graded Excellent for cut, in a setting that leaves the pavilion open to the light, cleaned once a fortnight. If you want a different shape, buy the different shape and spend the difference on the cutting rather than on the last tenth of a carat. Nobody has ever looked at a ring and counted facets. They notice whether it is alive.Comparing two stones and cannot pick?Send us both reports and the videos. We will tell you which one will look brighter on a hand, what each is worth here, and whether either has been cut to keep weight.
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