Cut: The C You Can
Actually See
Colour and clarity are graded under a loupe. Cut is graded by how the stone handles light, and that is the thing you notice from across a room. This page explains what the grade measures, then lets you change the angles of a diamond and watch where the light goes. Natural or lab-grown, the physics is identical.
Cut is not shape. Round, oval and pear are shapes. Cut is how well the cutter turned the rough into that shape: the angles, the proportions, the symmetry and the polish. Two round diamonds of the same weight, colour and clarity can look completely different, and cut is the reason.
A diamond does not glow. Every bit of light you see in it came in from the room, bounced around inside, and came back out towards your eye. The angles decide whether it comes back out the top, where you are looking, or slips out of the bottom and the sides, where you are not. A well-cut stone returns most of it. A badly cut one leaks, and no amount of colour or clarity will rescue it.
The GIA grades round brilliants Excellent, Very Good, Good, Fair or Poor by measuring proportions, symmetry and polish and comparing them against how real stones perform. Excellent is the floor for an engagement ring. It is also the one grade where you should refuse to trade down to pay for something else, because it is the only one you can see from arm's length.
This is the one C where natural and lab-grown are not just graded the same but behave the same. Light does not know how the stone was made. If anything, cut matters more when you buy lab-grown: colour and clarity come cheap there, so cut is where two lab-grown stones actually differ, and the one place to spend.
What you are looking at
A diamond sliced in half, seen from the side. The wide flat top is the table, the slope below it is the crown, the thin band is the girdle, and the pointed bottom half is the pavilion. You are the eye at the top, looking down at the ring.
Each thin line is a ray of light we follow into the stone. Gold lines bounce around and come back up to your eye: that is the sparkle you see. Red lines slip out of the bottom or the side: that is light you never see, and it is why a badly cut diamond looks dull or glassy however good its colour and clarity.
What to do: press Shallow, Ideal and Deep to see the three classic shapes, then drag the slider yourself and watch how many rays come back. The panel on the right tells you the grade a lab would give, how it would look from above, and how big a 1 carat stone would actually appear at that shape.
What the numbers on the certificate mean
A GIA or IGI report lists the proportions that produced the grade. You do not need to memorise them. You need to know the band that experienced cutters aim for, so that when a dealer says "it's an Excellent", you can ask which Excellent. The grade is a range, and the best stones sit in the middle of it.
The two that matter most are pavilion angle and total depth. Pavilion angle is the slider above. Depth is the stone's height as a percentage of its width, and it is where carat weight goes to hide. More on that below.
Cut grade as a GIA grade exists only for round brilliants. For ovals, cushions, pears and the rest there is no GIA cut grade at all, which is why the shape guide spends so long on proportions and why you must see a fancy shape in person.
| Measure | Aim for | Why |
|---|---|---|
| Pavilion angle | 40.6° to 41.2° | Decides whether light returns or leaks. The narrowest target of all. |
| Crown angle | 33° to 35° | Works with the pavilion. Drives fire. Too flat and the stone looks glassy. |
| Total depth | 59% to 62.5% | Outside this band the stone is either leaking light or hiding weight. |
| Table | 54% to 58% | Bigger tables look brighter but lose fire. Smaller tables trade the other way. |
| Girdle | Thin to slightly thick | Very thin chips. Very thick hides weight and can show as a grey ring. |
| Culet | None or very small | A large culet reads as a dark dot through the table. |
| Polish and symmetry | Excellent or Very Good | Both are on the report. "Triple Excellent" means cut, polish and symmetry all Excellent. |
Where cut and carat collide
Carat is weight, not size. A deep stone carries weight in its pavilion, below the girdle, where it does nothing for how big the diamond looks and actively hurts how bright it is. A well-proportioned 0.90ct will often look the same size across as a deep 1.00ct, and it will look better doing it.
This is the quiet way a cheap-looking 1 carat is sold: the carat is real, the price is tempting, and the depth is 66%. The readout above shows the spread, the diameter a 1 carat stone would have at each depth. Use it. The size guide shows what the difference looks like on a hand.
- 1.00ct at 56% depth, shallowabout 6.6mm across
- 1.00ct at 61% depth, idealabout 6.45mm across
- 1.00ct at 66% depth, deepabout 6.3mm across
- 0.90ct at 61% depth, idealabout 6.2mm across
Three things to ask for in the shop
The proportions, not just the grade. Ask for the pavilion angle, the depth and the table. If the dealer cannot tell you or will not show you the report, that is your answer.
Normal light. Jewellers' spotlights make every diamond sparkle. Ask to see it by the window, under the ordinary ceiling lights, and in your hand at arm's length. That is the light it will live in.
Two stones side by side. An Excellent next to a Good of the same weight is the fastest education you will ever get. Any jeweller confident in their stones will do this gladly. Some Hatton Garden dealers can also show an ASET or Idealscope image, a colour map of where the stone's light is coming from. Worth asking.