Diamond Cut Grade: The Complete Australian Guide

What is a diamond cut grade?

Cut is the only one of the 4Cs of diamond quality that a person controls. Colour, clarity and carat come from the rough crystal; cut is the work of the cutter. The cut grade is a laboratory's judgement of how well that work was done, based on how the finished diamond handles light.

A lab looks at three kinds of light return. Brightness is the white light reflected back to your eye. Fire is the flashes of rainbow colour created when light splits as it passes through the facets. Scintillation is the pattern of light and dark sparkle you see when the stone, the light or your eye moves. A well-cut diamond balances all three.

The cut grade is not the same as the shape. 'Round', 'oval' or 'princess' describes the outline; the cut grade describes the quality of the faceting within it. The term also does not apply to older styles such as the rose cut, which was designed for a soft glow rather than modern brilliance; how rose and brilliant cuts differ explains why. Any unfamiliar word on this page is also defined in our jewellery glossary.

What are the diamond cut grades from Excellent to Poor?

GIA (the Gemological Institute of America) introduced its cut grade for standard round brilliant diamonds in 2006, and it is now the scale most Australians see. Other labs use similar but not identical scales:

The names line up roughly, not exactly. An IGI 'Ideal' and a GIA 'Excellent' cover similar ground, but the labs set their boundaries differently, so it is worth reading comparing grades from different laboratories before you weigh two reports against each other.

GIA's overall grade also takes in more than light. It considers weight ratio (whether the stone carries extra weight that does not add to its face-up size), durability (mainly how thin the girdle, the narrow edge around the widest point, is), and the polish and symmetry grades. A stone with excellent proportions can still be held back to Very Good if, for example, its girdle is thick.

Excellent vs Very Good vs Good: which cut grade should you choose?

Most diamonds sold for engagement rings in Australia are Excellent or Very Good. Here is how each grade behaves, and when each is a sensible pick:

The gap between Excellent and Very Good is the one most people agonise over. A Very Good stone that misses Excellent by a fraction of a degree on one angle can look identical; one that misses on several measurements at once can look a little flat. That is why the numbers matter more than the label. Some retailers also sell a tighter 'Ideal' tier above Excellent, compared in Ideal versus Excellent cut diamonds.

How do proportions decide a diamond's cut grade?

A round brilliant has a crown (the top section above the girdle), a table (the large flat top facet), a pavilion (the cone underneath) and sometimes a culet (a tiny facet at the very bottom). The angles and percentages between these parts decide where light goes.

The pavilion angle is the single most important number. Too shallow (well below about 40.6°) and light leaks out the bottom, giving a washed-out ring called a fisheye. Too deep (well above about 41.8°) and light escapes out the sides, leaving a dark centre known as a nailhead. Crown angle and pavilion angle also work as a pair: a slightly steeper crown can balance a slightly shallower pavilion.

Depth affects size too. A 1.00 ct round cut to good proportions measures about 6.4–6.5 mm across. Cut it deep and the same weight may face up at 6.2 mm; cut it shallow and it may reach 6.6 mm but lose brightness. Our guide to round proportions beyond the grade goes into the finer combinations.

What do polish and symmetry grades mean?

Every GIA report lists polish and symmetry separately, each graded Excellent, Very Good, Good, Fair or Poor. Polish describes the surface finish of the facets: fine polishing lines or small burn marks lower it. Symmetry describes how precisely the facets are shaped, aligned and placed: a table that sits off-centre or facets that do not meet neatly lower it.

A round graded Excellent for cut, polish and symmetry is often called Triple Excellent or 3EX. It is a useful shorthand, but it is not a separate lab grade. Small dips in polish (Very Good instead of Excellent) are almost never visible without magnification; symmetry problems are more likely to show as uneven sparkle.

Do fancy shapes and step cuts get a cut grade?

No, not from GIA. GIA gives an overall cut grade only to standard round brilliants. Ovals, pears, cushions, emeralds and every other shape receive polish and symmetry grades only, because there is no single 'correct' set of proportions for them. A long oval and a short oval can both be well cut. Our page on judging cut quality in fancy shapes covers how to assess them without a grade.

For a fancy shape, you judge cut by its length-to-width ratio, depth and table percentages, the visible outline, and above all how it looks in video: watch for a dark bow-tie, windows (see-through patches) and dead zones. Square brilliants such as the princess cut work best with a depth of roughly 64–75% and a table of about 63–75%.

A rectangular step cut diamond, such as an emerald cut or a baguette, works differently again. Step cuts have long, parallel rows of facets that create broad flashes of light, sometimes described as a hall of mirrors, rather than pinpoint sparkle. They show inclusions and windowing more easily, so clarity and an even depth matter more. Emerald cuts are usually chosen at a ratio of about 1.30–1.50; baguettes range from roughly 1.5:1 to 3:1.

Side-stone and speciality step cuts follow the same logic: look for crisp, even steps and no window in the centre. That applies to bullet-shaped side stones, trapezoid side stones, triangular cuts, five-sided pentagonal stones and six-sided hexagonal cuts. Drop-shaped stones faceted all over, such as the briolette, are judged on even facets and an unbroken outline, since they have no table at all.

What are precision cut and premium cut diamonds?

Precision cut diamonds and premium cut diamonds are retail terms, not laboratory grades. No major lab prints 'precision' or 'premium' on a report. Sellers use them for stones they consider better than the average Excellent, such as a tight 'super ideal' proportion set, strong optical symmetry or a hearts and arrows pattern (eight hearts and eight arrows visible through a special viewer in a precisely cut round).

Some of these stones genuinely perform at the top of the Excellent range. The way to tell is the same for every diamond:

• Ask for the report: a GIA or IGI cut grade plus the full measurements.
• Check the angles: crown and pavilion angles against the proportions table above.
• Ask for light-performance images: ASET or Ideal Scope images show where light is returned or lost.
• Watch a video: even sparkle across the whole stone, no dark centre.

'Precision cut' is also used loosely for modern patented and speciality facet designs, many with extra or rearranged facets. The star cut, the multi-faceted Portuguese cut and the square Flanders cut are examples. These are judged by eye and by polish and symmetry rather than an overall cut grade.

How much does cut grade change a diamond's price?

Cut grade moves price less than carat or colour, but the step is real. As a rough guide for a 1.00 ct round, G colour, VS2 clarity, with a recognised report:

Prices vary with the exact proportions, fluorescence, the lab and the market, so treat these as ranges, not quotes. Laboratory-grown diamonds are now mostly cut to Excellent or Ideal, because growers can choose the crystal size to suit the cut, so a lower cut grade rarely saves much in lab-grown. Natural diamonds are more often cut to keep weight, which is where Very Good and Good stones come from.

Natural and lab-grown diamonds are graded on the same cut scales and look identical to the eye; lab-grown costs much less, natural is rarer. If you are balancing cut against other grades, whether VS1 and VS2 look different and when an SI1 makes sense show where you can save without losing sparkle.

How to check a diamond cut grade before you buy: step by step

1. Confirm the lab and the grade. Note who graded it (GIA, IGI or another lab) and the cut, polish and symmetry grades. For a fancy shape, expect polish and symmetry only.
2. Read the proportions. Find the table %, depth %, crown angle and pavilion angle on the report's diagram and compare them with the ranges above.
3. Check the diameter against the weight. A well-cut 1.00 ct round should be about 6.4–6.5 mm across; much less suggests a deep stone.
4. Look at the girdle and culet. Thin to slightly thick girdle and no culet or a very small one.
5. Watch it move. In a 360° video or in person, look for even sparkle, no dark centre, no glassy ring and, in fancy shapes, no heavy bow-tie.
6. Weigh the trade-offs. Keep the cut high and adjust colour, clarity or carat first if you need to meet a budget.