Diamond Education
Four Words That Set the Price, and One That Sets the Sparkle
Every diamond is described by the same four measures: cut, colour, clarity and carat. They are what a grading laboratory reports and what a price is built from.
They are not equally important. Cut decides how a diamond behaves in daylight, and it is the only one of the four that a cutter controls rather than nature. Colour and clarity mostly decide what you pay, not what you see, once you are past a certain point. Carat is the one everybody starts with and the one worth deciding last.
What follows is the order we would use ourselves.
In Order of What You Will Notice
The Four Cs, Ranked
If you only take one thing from this page: cut is the one to protect, and clarity is the one people most often overpay for.
- Cut, and it is not the shape
Cut describes proportion and finish: how well the facets are placed to gather light and return it. It is the difference between a diamond that lifts across a room and one that looks flat under the same lamp.
Confusingly, cut is also used to mean shape, as in emerald cut. On a grading report, cut means quality of cutting. Aim for Excellent on a round brilliant, and accept nothing below Very Good.
- Colour, which means absence of it
White diamonds are graded D to Z, D being colourless and Z showing an obvious tint. The scale is finer than the eye is: most people cannot separate D from G with the stone in a ring.
Metal matters more than the letter. A G looks icy in platinum and warm in yellow gold, and a J that would look tinted in platinum looks perfectly white set in 18ct yellow.
- Clarity, at arm's length
Clarity records the inclusions formed as the diamond grew. Grades run from Flawless down to I3, and almost all of that scale is invisible without a loupe.
What matters is whether you can see anything at normal distance, and whether an inclusion sits somewhere structurally unwise. VS2 and a well-placed SI1 are the value grades, and always worth looking at rather than buying by letter.
- Carat, which is weight and not size
A carat is a fifth of a gram. Two diamonds of the same weight can look quite different: a deep stone hides its weight in the bottom, a well proportioned one puts it across the top where you see it.
Prices step up sharply at the round weights, so 0.90ct often buys a visibly better diamond than 1.00ct for the same money, and nobody looking at the ring will know.
Cut
What the Cut Grades Mean in Practice
Cut grades are only reported on round brilliants. Fancy shapes such as oval, pear and emerald are graded for polish and symmetry instead, which is why proportions and a good look at the stone matter more on those.
| Grade | What you see | Worth buying |
|---|---|---|
| Excellent | Maximum brilliance and fire, the light return the shape is capable of | Yes, on a round brilliant |
| Very Good | Very close to Excellent, and often better value | Yes, worth comparing side by side |
| Good | Noticeably less lively, particularly in low light | Only with a reason |
| Fair | Visibly dull next to a better cut stone | No |
| Poor | Light leaks out of the bottom of the stone | No |
On fancy shapes we look at length to width ratio, the shape of the outline, and whether a bow tie shadow runs across the middle. None of that appears as a single grade on a report.
Colour
The Colour Scale, Grouped as You Would Actually Shop It
Laboratories grade in single letters, but the eye works in bands. These are the groups worth thinking in, and the metal you would pair each with.
- D to FColourless. Icy white, the premium grades, best in platinum and white gold
- G to HNear colourless. Looks white in any metal. The value sweet spot
- I to JNear colourless with a hint of warmth. Excellent in yellow and rose gold
- K to MFaint warmth, visible in daylight. Can be lovely in yellow gold, deliberately
- N to ZClearly tinted. Rarely what someone means by a white diamond
The swatches are indicative of relative warmth on screen and are not a grading tool. Colour is graded face down against masterstones in controlled light, which is not a judgement anyone can make from a photograph.
Clarity
Where Clarity Stops Being Visible
Everything from VS2 upward looks identical in a ring. The money saved by not buying VVS on a stone nobody will loupe is better spent on cut, or on size.
| Grade | What you can see | Who it suits |
|---|---|---|
| FL and IF | Nothing, even at ten times magnification | Rarity, not appearance |
| VVS1 and VVS2 | Extremely hard to find under a loupe | Very high specification stones |
| VS1 and VS2 | Visible under a loupe, invisible to the eye | Most people, most of the time |
| SI1 | Often eye clean, sometimes not. Look at the stone | Best value when it is clean |
| SI2 | Sometimes eye clean, frequently not | Only after seeing it |
| I1 to I3 | Inclusions visible without magnification | Not for a centre stone |
Position matters as much as grade. An inclusion under a claw is hidden for life; the same inclusion in the centre of an emerald cut, which has large open facets and nowhere to hide, is a different proposition. We look at every centre stone before recommending it.
Carat
What the Weights Look Like
Approximate face up diameters for a well proportioned round brilliant. Fancy shapes of the same weight read larger or smaller depending on how the weight is distributed: an oval or a marquise of a given carat looks bigger than a round, and a cushion often looks smaller.
- 0.25ct
-
About 4.1mm across
- 0.50ct
-
About 5.2mm across
- 0.70ct
-
About 5.8mm across
- 0.90ct
-
About 6.3mm across, and priced well below a full carat
- 1.00ct
-
About 6.5mm across
- 1.50ct
-
About 7.4mm across
- 2.00ct
-
About 8.1mm across
- 3.00ct
-
About 9.3mm across
Two thirds of a carat well cut will outshine a full carat cut badly, every day of the week. If size is the priority, say so and we will show you where the weight is best spent.
How We Would Spend It
Six Decisions That Move the Money
- Protect the cut gradeIt is the only C you cannot compensate for later, and the only one that changes how the ring looks across a table.
- Buy just under the round weights0.90ct instead of 1.00ct, or 1.40ct instead of 1.50ct, is usually a better stone for the same figure.
- Let the metal choose the colour gradeGoing below H in platinum shows. In 18ct yellow gold, I and J look white and cost noticeably less.
- Stop paying for clarity at VS2Above that, you are buying a certificate rather than an appearance.
- Insist on a report for a centre stoneGIA or IGI, at any meaningful size, natural or lab-grown. It is how the stone is identified and insured for the rest of its life.
- Look at the shape, not only the shape nameTwo ovals of identical specification can differ a great deal in outline. This is the part no report captures.
Our Position
A Report Is Evidence, Not a Recommendation
Two diamonds with identical reports can look quite different, and the paperwork will not tell you which is which. Light return, the shape of an outline, whether a bow tie shadow crosses the centre of an oval: none of it appears as a grade.
So we look at stones before we put them in front of you, and when a decision is close we will show you both and explain what we think and why. If you would rather see them yourself, we can arrange a face to face meeting.
Put It Against Your Budget
See the Trade-Offs on Real Stones
The Four Cs make sense the moment they have numbers attached. Tell us your budget, the shape you like and the metal you have in mind, and we will send a short list with the compromises spelled out in each direction.
Common Questions
Four Cs Questions
Which of the Four Cs matters most?
Cut, without much argument. It decides how the diamond behaves in light, and it is the only one of the four determined by the cutter rather than by how the stone formed. A modest stone cut beautifully looks better than a larger one cut poorly.
What does eye clean mean?
That you cannot see any inclusions with the naked eye at normal viewing distance, roughly arm's length, without magnification. Many SI1 diamonds are eye clean and cost considerably less than VS grades. It is a description of the individual stone rather than a grade, which is why it needs looking at rather than reading.
Is a D colour diamond worth the premium?
Only if the difference matters to you knowing it is there, because it will not be visible in the ring. Most people cannot distinguish D from G once a stone is set, and in yellow or rose gold the metal warms the appearance anyway. G and H give you white without the premium.
Why is 0.90ct so much cheaper than 1.00ct?
Because prices step at the round weights, where demand concentrates. The visual difference between 0.90ct and 1.00ct is about a fifth of a millimetre across the top, which nobody can see. The price difference is substantial and can be spent on cut or colour instead.
Do the Four Cs apply to lab-grown diamonds?
They have done, and largely still do in practice. In October 2025 GIA moved lab-grown reporting to a Premium or Standard framework rather than the traditional letters, but most lab-grown stones on the market still carry earlier GIA reports or reports from IGI and other laboratories using the familiar wording. We explain which framework a given report uses.
Do the Four Cs apply to coloured gemstones?
Not in the same way. With a sapphire, ruby or emerald, colour is overwhelmingly the driver of both beauty and price, and inclusions are judged by type rather than on the diamond clarity scale. Emeralds in particular are almost always included and are graded on that expectation.
Should a small accent diamond have a report?
No, and it would cost more than the stone. Reports are for centre stones and any stone of meaningful individual value. Accent and shoulder diamonds are specified by quality range instead, and we document that range with your order.
The Diamond and Gemstone Guide
Free Download
The Diamond and Gemstone Guide
The Four Cs in full, plus shapes, natural against lab-grown, fancy colour, coloured gemstones, treatments, durability and how to read a grading report.