Four measurements, one stone
The four Cs are cut, colour, clarity and carat. Together they describe a diamond precisely enough that two people who have never seen the stone can agree on what it is. They apply to lab-grown diamonds exactly as they apply to mined ones.
They are not equally important. If you remember one thing, remember that cut comes first.
Cut
Cut is the only C decided by a person. It describes how well the facets of a diamond are proportioned, aligned and polished, and so how much light the stone returns to the eye. A well cut diamond is bright across its whole face. A poorly cut one has dark or glassy areas, whatever its other grades.
Laboratories grade the cut of round brilliants from Excellent down to Poor. IGI also uses Ideal at the top. A round brilliant has 57 facets, or 58 when the point at the bottom, the culet, is polished flat.
What to choose. Excellent or Ideal. This is the last place to save money.
Colour
Colour measures how much tint a white diamond has. The scale runs from D, which is colourless, to Z, which is visibly yellow or brown.
| Grade | Described as | What you see |
|---|---|---|
| D, E, F | Colourless | No tint, even beside a whiter stone |
| G, H, I, J | Near colourless | White when set. A trace of warmth when compared loose |
| K, L, M | Faint | A soft warmth that many people like in yellow gold |
| N to Z | Very light to light | Visible tint |
What to choose. G or H looks white in any setting. In yellow or rose gold, the metal lends its own warmth to the stone and a lower grade is hard to tell apart.
Clarity
Clarity measures the small marks inside a diamond, called inclusions, and on its surface, called blemishes. It is graded under ten times magnification.
| Grade | Meaning |
|---|---|
| FL, IF | Flawless and internally flawless. Nothing to see at ten times magnification |
| VVS1, VVS2 | Very, very slightly included. Difficult for a grader to find |
| VS1, VS2 | Very slightly included. Minor, and not visible to the eye |
| SI1, SI2 | Slightly included. Noticeable under magnification, sometimes to the eye |
| I1, I2, I3 | Included. Visible to the eye |
What to choose. A diamond that is clean to the eye. That is usually VS2 or better, and often SI1. Step cuts such as emerald and Asscher show inclusions more readily, so go one grade higher.
Carat
Carat is weight, not size. One carat is 0.2 grams, and it is divided into 100 points, so a 0.50 carat diamond is a fifty pointer.
Two diamonds of the same weight can look different in size, because a deep stone hides weight below its edge. For a well cut round brilliant, weight and diameter line up like this.
| Carat weight | Approximate diameter |
|---|---|
| 0.25 ct | 4.1 mm |
| 0.50 ct | 5.1 mm |
| 0.75 ct | 5.8 mm |
| 1.00 ct | 6.4 mm |
| 1.50 ct | 7.4 mm |
| 2.00 ct | 8.1 mm |
| 3.00 ct | 9.3 mm |
| 4.00 ct | 10.2 mm |
| 5.00 ct | 11.0 mm |
What to choose. Prices step up at round numbers. A 0.90 carat stone looks almost the same as a 1.00 carat stone.
Which C matters most
In order: cut, then carat, then colour, then clarity.
Cut makes a diamond bright. Carat decides how large it looks. Colour only matters where it can be seen, and clarity only matters where it can be seen without a loupe. Spend on what the eye notices.
A lab-grown diamond costs less than a mined diamond of the same grade, which is why an excellent cut and a larger carat weight are usually both within reach. Read more in lab-grown diamonds, explained.
