The 4Cs of a diamond

Four things decide what a diamond is worth, and they are not equally important. Understanding which is which is the difference between paying for something you can see and paying for something only a certificate knows about.

Carat is weight, not size

A carat is a fifth of a gram. It is a measure of how heavy a diamond is, not how big it looks, and the two come apart more often than people expect: a shallow-cut stone spreads wider for the same weight, a deep one hides its weight underneath.

Weight also does not rise in a straight line with price. A two carat diamond costs far more than twice a one carat, because large rough is rarer than small rough. The steepest jumps sit exactly on the round numbers — one carat, two carats — which is why a 0.90 ct stone can look identical to a 1.00 ct and cost noticeably less.

Cut is the one you can see

Cut is not the shape. Round, oval, emerald, pear — those are shapes. Cut is how well the stone has been proportioned and finished, and it is the only one of the four that decides how much light comes back out of it.

It is also the one worth spending on. A poorly cut diamond of excellent colour and clarity looks dull, because light entering the top leaks out of the bottom instead of returning to the eye. A well cut stone one grade lower in colour will outshine it in any room either of them is ever worn in.

Colour is graded by absence

Diamond colour is measured by how little of it there is. The scale runs from D, which is colourless, down through the alphabet as a faint yellow or brown becomes perceptible.

Every lab-grown diamond DLV sets is E or F. Both sit in the colourless band, at the top of the scale, and the difference between them is not something the eye finds outside a grading laboratory — a stone is compared face-down against masterstones under controlled light, which is not the way anybody looks at a ring.

Clarity is about what is inside

Almost every diamond contains something: a trace of uncrystallised carbon, a tiny fracture, a mark from how it grew. Clarity grades how visible those are under ten times magnification.

The practical line is "eye-clean" — whether anything can be seen without a loupe. VS stones, which is what we set, are eye-clean. Paying for a grade above that buys a difference that only appears under magnification.

Where the money is best spent

If a budget has to bend, bend it on carat and clarity rather than cut.

• Cut first. It decides how the diamond behaves in light, which is the whole point of a diamond.
• Then colour, but only down to the colourless band. Beyond that the return falls away.
• Clarity only needs to reach eye-clean.
• Carat last, and consider stepping just under a round number.

What this means for a lab-grown diamond

Nothing changes. A lab-grown diamond is graded on the same four properties by the same laboratories using the same scales, because it is the same material — carbon in the same crystal structure, with the same hardness and the same optical behaviour. What changes is how much of the scale you can afford.