Carat – the weight
Carat is a unit of weight, not of size: 1 carat (ct) = 0.2 grams, divided into 100 points. A 0.50 ct stone weighs 0.1 grams. How large a stone looks depends on the cut as well as the weight: a shallow stone shows more surface, a deep one hides weight in its depth. That is why every report also states the measurements in millimetres. A 1.00 ct round brilliant typically measures about 6.4 to 6.5 mm in diameter.
Colour
GIA's colour scale starts at D (completely colourless) and ends at Z (distinctly yellowish or brownish). The letters A to C were deliberately skipped to set the scale apart from older systems. The stone is graded pavilion-up under standard light against master stones.
| Grade | Group | What you see |
|---|---|---|
| D, E, F | colourless | No colour detectable, even to experts; differences only in direct comparison. |
| G, H, I, J | near colourless | Colourless from above; a slight tint is detectable from the side or against D–F. |
| K, L, M | faint | A slight warm tint is visible from above too; often unobtrusive set in yellow gold. |
| N and lower | tinted | Clearly perceptible colour. |
Coloured diamonds (fancy colours – pink, blue, yellow and others) are not graded on this scale but described by hue, saturation and tone, for example "Fancy Light Pink".
Clarity
Clarity describes how many and how visible inclusions (inside) and blemishes (outside) a stone shows under 10× magnification. Size, number, position, nature and visibility are assessed.
| Grade | Meaning | In practice |
|---|---|---|
| FL | Flawless – free inside and outside | Extremely rare. |
| IF | Internally Flawless – free inside | Only external blemishes, e.g. minor polish marks. |
| VVS1, VVS2 | Very, Very Slightly Included | Inclusions hard to find under the loupe even for experts. |
| VS1, VS2 | Very Slightly Included | Visible under the loupe, as a rule not to the naked eye. |
| SI1, SI2 | Slightly Included | Easy to find under the loupe; at SI2 and with large tables sometimes visible to the naked eye. |
| I1, I2, I3 | Included | Visible to the naked eye; at I2/I3 inclusions can affect transparency and durability. |
"Eye-clean" is not a laboratory grade but a trade description: no inclusions visible without a loupe. In step cuts (emerald, Asscher) with their large table, inclusions show earlier than in brilliant cuts.
Cut
Cut covers three things: the shape (round brilliant, oval, emerald …), the proportions (table size, depth, crown angle, pavilion angle) and the execution (polish and symmetry). It is the only one of the four Cs determined by people, and it decides how much light the stone returns.
GIA assigns an overall cut grade to round brilliants only: Excellent, Very Good, Good, Fair, Poor. For all other shapes the report states polish and symmetry on the same scale, plus the proportions in percent. Guide values for round brilliants are a table of 54–60% and a total depth of 59–63%.
The fifth C: the certificate
Without a report the four Cs are claims. The report of an independent laboratory (GIA, IGI, HRD Antwerp) makes them verifiable: the report number can be looked up online with the laboratory, the measurements checked, the laser inscription read. Our article on verifying a GIA report shows how.
Reading measurements and proportions
Besides the four Cs the report states the measurements in millimetres and the proportions in percent. The measurements tell you how large the stone actually is: two 1.00 ct round brilliants can measure 6.3 and 6.6 mm in diameter, depending on how deep they are cut. Depth in percent (total height divided by diameter) and table in percent (table width divided by diameter) describe the ratio. A stone cut too deep hides weight; one cut too shallow lets light escape through the bottom – in both cases brilliance drops, which shows in the cut grade of a round brilliant.
Common misconceptions
- "More carats = bigger": only at the same cut. The measurements in millimetres say more about visible size.
- "D is the best colour": D is the most colourless grade. Whether G or D pleases more you only see in comparison; set in yellow gold, differences between D and H are as a rule not detectable.
- "VVS is loupe-clean": loupe-clean (IF or LC) means no inclusions under 10× magnification; VVS has some that are very hard to find.
- "Excellent cut applies to all shapes": the GIA cut grade exists for round brilliants only; for other shapes polish and symmetry are stated.
- "The report states the value": it states properties. Prices form in the trade from many factors.
How the 4Cs interact
The four criteria are independent of each other; there is no "right" stone, only priorities. Anyone who wants sparkle above all looks first at Cut Excellent in a round brilliant. Anyone choosing a step cut looks first at clarity. Anyone planning a yellow-gold setting can start lower on the colour scale than with platinum. And anyone who wants to compare entries needs a report of the same type for each stone – only then does "G VS1" mean the same for two stones.
In our configurator all four Cs can be set as filters; every entry in the shop corresponds to the line on the stone's report.