Carat is the easiest of the four Cs to understand and the easiest to over-weight in a decision. It records what a stone weighs, nothing else. It says nothing about how bright it is, how clean it is, or how large it looks once it is on a finger.

Weight against spread

Spread is the width of a diamond across the top, and it is what anyone actually sees. A well proportioned one carat round brilliant measures around 6.4mm across. A stone of the same weight cut too deep will carry more of its mass hidden below the setting and can look closer to a 0.85. Cut too shallow and it will look wide but lose light through the bottom.

This is why we would rather sell you an excellent cut at 0.90 carat than a mediocre one at a full carat. On the hand, the first usually looks bigger, and it certainly looks brighter.

The prices jump at round numbers

Diamond pricing is not smooth. It steps up sharply at the weights people ask for, 0.50, 0.70, 1.00, 1.50 and 2.00 carats, because demand clusters there. The stone does not change at those thresholds. Only the price does.

A 0.95 carat can cost meaningfully less than a 1.00, and the difference across the top is about a fifth of a millimetre, which nobody can see. If you are working to a budget, buying just under a threshold is the single most effective decision available to you.

Carat is not the same as size on the hand

• Finger size changes the impression. The same stone reads larger on a slim finger and smaller on a broader one.
• Shape changes the impression. Elongated cuts such as oval, marquise and pear cover more surface than a round of the same weight.
• Setting changes the impression. A slim band and fine claws leave more of the stone visible. A heavy setting can swallow it.

Total carat weight

Where a piece holds more than one stone, you will sometimes see a total carat weight, written as ct tw. That figure adds every stone together. Ten stones of 0.10 carat make one carat total, and they will never look like a single one carat diamond or cost like one. Always ask for the weight of the centre stone separately.

Our advice

Decide the cut grade first, the colour and clarity second, and let carat take whatever budget is left. A smaller stone that is cut properly outperforms a larger one that is not, in every light and at every angle, for as long as it is worn.