Carat, cut, color and clarity — the 4Cs — are a grading vocabulary, developed by the Gemological Institute of America so that a diamond could be described precisely on paper instead of by adjectives. They are four separate measurements of four separate things. A diamond does not have one 4Cs score; it has four independent readings, and this page describes what each one actually measures — no prices, and no opinion on what any of it is worth to a given buyer.
Carat — weight, not size
A carat is a unit of mass: exactly 0.2 grams, fixed internationally in 1907 so that “carat” would stop meaning slightly different things in different countries. It measures weight, not how large a stone looks. Two diamonds of the same carat weight can look different sizes depending on how they are cut — a deep, poorly proportioned cut hides weight in the bottom of the stone instead of spreading it across the top, where the eye actually judges size. The exact conversion is in the carat to gram converter, and how carat weight relates to millimetre size across different gem species is in the gemstone carat size chart.
Color — a scale that runs backwards from the alphabet
The GIA Color Scale runs from D to Z, deliberately starting at D rather than A so nobody could claim it as a fraction of some earlier, unrelated grading system. D is completely colourless; Z carries a noticeable yellow or brown tint. Grading is done by comparing a stone against a set of master stones under controlled lighting, and the scale groups into five bands: D–F (colorless), G–J (near colorless), K–M (faint), N–R (very light), and S–Z (light). Most of what a jeweller calls “colorless” to the naked eye actually spans several of these letters, since the differences at the top of the scale are subtle enough to need a trained grader and a controlled comparison to see at all.
Clarity — eleven grades, one magnification
Clarity grades how clean a diamond is of inclusions (internal features) and blemishes (surface features), judged under 10× magnification by a trained grader. The GIA scale has eleven specific grades across six named groups:
- FL (Flawless) — no inclusions or blemishes visible at 10×.
- IF (Internally Flawless) — no inclusions, blemishes only.
- VVS1, VVS2 (Very, Very Slightly Included) — inclusions difficult for a skilled grader to see at 10×.
- VS1, VS2 (Very Slightly Included) — minor inclusions, ranging from difficult to fairly easy to see at 10×.
- SI1, SI2 (Slightly Included) — inclusions noticeable to a skilled grader at 10×.
- I1, I2, I3 (Included) — inclusions obvious at 10×, and at the lower end visible without magnification, sometimes affecting transparency.
What separates one grade from its neighbour is the number, size, relief and position of the inclusions present — not any single feature on its own.
Cut — the only C that is about craftsmanship, not the rough stone
Carat, color and clarity all describe what nature put into the rough stone. Cut describes what a cutter did with it. GIA’s formal cut grade — Excellent, Very Good, Good, Fair, Poor — is defined specifically for the standard round brilliant, and is worked out from seven factors together: brightness, fire, scintillation, weight ratio, durability, polish and symmetry. Fancy-shaped diamonds (oval, pear, emerald and the rest) are described on a GIA report by their proportions rather than assigned this same five-point cut grade, since the geometry that defines a well-cut oval is not the geometry that defines a well-cut round.
Four numbers, not one
A grading report lists all four independently because they measure genuinely different things — a diamond can sit at the colourless end of the color scale and the included end of the clarity scale in the same breath, and neither number tells you anything about the other. Reading the four together is what a report is for; picking which combination suits a particular stone, budget or person is not something this page has a view on.