Cushion Cut Diamond Proportions: Ideal Chart and Light Guide

What are ideal cushion cut diamond proportions?

Proportions are the measurements that describe a diamond's shape in three dimensions: how deep it is, how big its top facet is, how thick its edge is and how its crown (the top part) relates to its pavilion (the lower, pointed part). If those parts are new to you, our guide to the table, crown, girdle, pavilion and culet explains each one with a diagram. In a cushion, these numbers decide whether light bounces back up to your eye or leaks out through the bottom and sides.

A cushion is a square or rectangular diamond with rounded corners and gently curved sides, a little like a pillow. It is cut in two main faceting styles, which our cushion cut buying guide covers in full. Because cutters shape each cushion to suit the rough crystal, proportions range widely, and GIA does not give cushions an overall cut grade the way it does for round brilliants. That makes a proportions chart more useful here than for almost any other shape.

Treat this chart as a starting filter, not a final verdict. Two cushions with identical numbers can look quite different, because the report does not record the angle of every facet. The numbers narrow your search; your eyes, a video and a reflector image make the final call. Any word in this guide you have not met before is also in our plain-English jewellery glossary.

What depth and table percentage should a cushion diamond have?

Total depth % is the height of the diamond from table to culet, divided by its width. Table % is the width of the large flat top facet, divided by the width of the stone. For a rectangular cushion, both are calculated against the shorter side, the width.

Depth matters most. Cushions are often cut deep, because a deeper stone keeps more weight from the rough crystal and weight is what sets the price. A cushion at 72% or more carries a lot of that weight underneath, where you cannot see it, so it faces up smaller than its carat suggests. Very deep cushions are also the ones most likely to show a dark centre. At the other end, a cushion under about 58% deep often lets light straight through the middle.

The difference looks small in millimetres, but the 62% stone has roughly a tenth more face-up area than the 74% stone at the same weight, and the eye notices area. A round brilliant of 1 carat is about 6.4 mm across, which is why cushions tend to look smaller for their weight; our round and cushion comparison sets out the numbers. For the table, 58–65% is a comfortable middle. A smaller table gives a larger crown and more coloured flashes (fire); a larger table gives more white brightness but flatter-looking faceting.

What is the best length-to-width ratio for a cushion diamond?

The length-to-width ratio is the length divided by the width, both taken from the report's measurements. A ratio of 1.00 is a perfect square. Cushions are sold from about 1.00 up to 1.50 or more, and the ratio you choose is mostly about taste, though it does change how the stone sits on the hand.

Elongated cushions have their own quirks, including a tendency to show a faint bow-tie, a dark band across the middle. Our guide to elongated cushions and their ratios goes into that in more detail. To see how the cushion sits among its square relatives, the princess, radiant and Flanders, read our overview of the square and cushion family of cuts.

Check the corners as well as the ratio. Matching corners, all four rounded by the same amount, matter more than whether the ratio is 1.01 or 1.04. A cushion with one sharper corner looks lopsided once it is set, and claws will draw the eye straight to it.

How do cushion cut diamond proportions affect windows, nailheads and fisheyes?

Diamond has a refractive index of about 2.42 and a critical angle of about 24.4°. In plain terms, light inside a diamond that hits a facet at a steep enough angle reflects back inside; light that hits it more squarely escapes. A well-proportioned pavilion is angled so that most light reflects twice off the lower facets and comes back out through the top. When the pavilion is too shallow or too deep, light escapes where it should not, and each mistake has a recognisable look.

• Window: a see-through, washed-out patch in the centre of the table. Caused by a pavilion that is too shallow, so light passes straight out the bottom. You can sometimes read text through it. Most common in cushions under about 58% depth.
• Fisheye: a grey or white ring inside the table, which is the reflection of the girdle (the thin outer edge). It also comes from a very shallow pavilion, usually combined with a large table. It is easiest to see from straight above.
• Nailhead: a dark centre, like the flat head of a nail, caused by a pavilion that is too deep. Light reflects off one pavilion facet and then leaks out the side instead of returning upward. Most common in cushions above about 70–72% depth.
• Extinction: dark areas that stay dark as the stone moves. Some dark contrast is normal and makes sparkle visible. Problem extinction is large, fixed patches, often in the corners or under the table.

Cushions are more prone to nailheads than most shapes, because of that commercial habit of cutting deep. Elongated cushions can also show a bow-tie, the same issue described in our oval proportions chart. Windows and fisheyes are less common but do turn up in shallow, spread-out cushions cut to look big for their weight.

How do you spot light problems in photos, video and ASET images?

A grading report cannot tell you whether a cushion has a nailhead or a window. You need to see the stone, or good images of it. Here is what to look for in each format:

• Still photos: useful for the outline and corners but easy to flatter with lighting. Look for a grey or glassy patch in the centre and a darker ring just inside the edge.
• 360° video: the most useful tool for most buyers. Watch the centre as the stone rocks. Sparkle that moves across the whole face is good; a centre that stays dark is a nailhead; a centre that stays pale and lifeless is a window.
• ASET image: the Angular Spectrum Evaluation Tool, developed by the American Gem Society, shows where light enters the diamond. Red marks bright light from high angles, green marks softer light from lower angles, blue marks contrast from your own head, and white or clear patches show leakage.
• Idealscope image: a simpler reflector. Red shows light return; white or pale areas show light leaking out; black shows contrast. In a good cushion the face is mostly red, with even black contrast and little white.

Look at symmetry in the reflector image too. The pattern should look much the same in each quarter of the stone. A pattern that is bright on one side and leaky on the other usually means the pavilion facets were cut at uneven angles, which no single number on the report will reveal.

Do chunky and crushed ice cushions need different proportions?

Yes, a little. A cushion brilliant has larger, bolder pavilion facets that give separate flashes of light, often called the chunky look. A cushion modified brilliant adds an extra row of small facets on the pavilion, which breaks the light into tiny pinpoints, the crushed ice look. Our page on chunky versus crushed ice faceting shows the difference side by side.

Crushed ice cushions are usually cut a little deeper, often in the 64–70% range, because the extra facets need more room. They hide inclusions well but can look slightly hazy if they are very deep, and they show colour more readily. Chunky cushions are often a touch shallower, around 60–66%, with a crisper pattern. Neither look has an official name on most reports; GIA uses 'cushion brilliant' or 'cushion modified brilliant', and the modified label can cover both chunky and crushed ice stones, so you have to judge from the image. More on the crushed look is in our guide to cushion modified crushed ice diamonds.

Antique styles follow different rules again. Old mine cuts, the cushion's ancestor, typically have a high crown, a small table (often under 55%) and an open culet you can see through the top. Those numbers would fail a modern chart, yet they are exactly what gives an old mine its soft, candlelit look. See our comparison of old mine and modern cushion cuts before you judge an antique stone by modern figures.

Do proportions change how much colour a cushion shows?

They do. Light travels further inside a deep stone, and the longer path picks up more body colour (the faint yellow or brown tint in a white diamond). The rounded corners and crowded facets of a cushion also tend to collect colour, especially in crushed ice styles. A 70% deep cushion can look a full colour grade warmer than a 63% stone graded the same.

In practice, if you are choosing a deep crushed ice cushion for white gold or platinum, many people go one grade higher in colour than they would for a round, for example G instead of H. In yellow or rose gold, the metal warms the stone anyway, and I or J often looks fine. Our article on why cushions can show more body colour explains the reasons in more depth.

How do cushion proportions compare with other diamond shapes?

Every shape has its own ideal range, because light behaves differently in each outline. Here is how the cushion sits against the other common shapes:

The radiant is the cushion's closest relative, with cut corners instead of rounded ones and similar depth figures; compare the numbers in our radiant diamond proportions chart and the look in radiant versus cushion, side by side. If you are torn between a cushion and a step cut, see Asscher and cushion compared and our emerald cut proportions chart, where windows matter far more. For a pointed shape, the pear diamond proportions guide sets out its own ranges.

Are lab-grown cushion diamond proportions any different from natural?

The ideal ranges are the same, because light behaves the same way in laboratory-grown diamond as in natural diamond: both have a refractive index of about 2.42 and a hardness of 10 on the Mohs scale. The difference is in how they tend to be cut. Natural rough is expensive, so natural cushions are more often cut deep to save weight. Lab-grown rough costs far less, so cutters have less reason to save weight and lab-grown cushions are more often cut closer to the middle of the chart, though deep and shallow lab stones still exist. Always check the numbers either way.

These are broad ranges only; prices move with the market and with the exact grades. Within each type, a well-proportioned cushion does not always cost more than a deep one of the same weight, which is why checking proportions is one of the most useful things a buyer can do. Both natural and lab-grown cushions should come with an independent report, usually from GIA or IGI, that states clearly whether the stone is natural or lab-grown.

How to check a cushion diamond's proportions: step by step

1. Read the report. Note the measurements, total depth %, table %, girdle, culet, polish and symmetry, and whether the stone is natural or lab-grown.
2. Filter by depth and table. Start with depth 61–67% and table 58–65%; widen to the very good range only if the stone looks right.
3. Work out the ratio. Divide length by width. Aim for 1.00–1.05 for square or 1.10–1.35 for elongated, then check the four corners match.
4. Compare face-up size. Look at the millimetre measurements, not just the carat weight, so a deep stone does not surprise you.
5. Watch a 360° video. Look for a dark centre (nailhead), a glassy centre (window) or a grey ring (fisheye).
6. Ask for a reflector image. An ASET or Idealscope image should show even light return across all four quarters with little white leakage.
7. Match colour to depth and style. For deep or crushed ice cushions in white metal, consider one colour grade higher.