The 4Cs

The 4Cs are four questions a grader answers about a polished diamond. Cut: how well do the facets handle light? Colour: how little tint does the stone show? Clarity: how free is it of inclusions inside and blemishes on the surface? Carat weight: how much does it weigh?

The name goes back to the early 1940s, when GIA's founder, Robert M. Shipley, coined it as a memory aid. The grading system that established the four factors was developed in the 1950s, and it is now the shared language jewellers and laboratories use to describe a stone. Colour, clarity and carat weight were also the basis of the first diamond grading system, set up in India more than 2,000 years ago. Used together, the four describe a finished diamond, and GIA ties its value to how rare each quality is.

Each C has its own entry: cut, colour, clarity and carat weight.

GIA's own answer is that it depends on your taste and budget, and it links a diamond's value to how rare each C is rather than naming one winner. Still, the four behave differently for a buyer.

Cut is said to have the greatest effect on a stone's beauty. An expertly cut diamond can look brilliant despite lower colour and clarity grades, while a poorly cut stone with top grades can look dull. Carat weight is the most objective C, because it is read from a calibrated digital scale, according to the American Gem Society (AGS). Colour and clarity are judgments made against reference standards, and many colour distinctions are too subtle for an untrained eye.

One workable order for a ring is to settle on cut quality first, then the carat weight you want, then the lowest colour and clarity grades that still look white and clean to you. It is a default, not a rule. If you have your heart set on one shape, that choice can reorder the list.

The Cs are graded separately but seen together, so a change in one can change how another looks.

• Shape, colour and clarity. Brilliant cuts tend to play down both tint and small flaws because of their many facets and sparkle. Step cuts such as the emerald cut have larger facets that show colour and clarity more readily, so a buyer who wants a white, eye-clean step cut may need higher grades.
• Shape and size. Elongated shapes such as the oval cut can give the illusion of greater size than a round diamond of the same carat weight.
• Weight and price. With the other three Cs equal, a 1-carat diamond costs more than twice as much as a half-carat one. Weight is not priced in a straight line.
• Neighbouring stones. In an all-diamond three-stone ring, GIA suggests keeping the side stones within about two colour grades of the centre stone, because much whiter side stones make the centre stone look more yellow by comparison.

The 4Cs are a summary, not a full description of one stone. A grading report from a laboratory such as GIA records the grades for a specific diamond. GIA's cut grade also draws on polish and symmetry, plus weight ratio and durability, none of which appear in the four headings. Two stones with identical 4Cs can still look different side by side.

AGS calls informal terms such as “eye clean” vague, and recommends having a diamond graded by a credentialed expert or a reputable laboratory. Whether a stone is natural or lab-grown is a separate decision from its grades.

Start with a budget and a shape, then use the 4Cs to compare actual stones instead of chasing the top grade in every column. Read the report, then look at the stone, ideally in the kind of setting you plan to use.

If you are planning a custom engagement ring, the design studio lets you note your preferred shape, metal and stone first, and a jeweller confirms the specifics. You can browse engagement rings for ideas, or talk it through at one of the showrooms.