The four Cs: which weighs heaviest when selling?

Two that are fixed, two that remain a judgement

Carat is the weight and is the only C there is nothing to argue about. A carat is exactly 0.2 grams, and the trade counts in hundredths of one. A five carat stone weighs one gram, which is immediately why a small box can hold a serious sum.

Cut is measurable with instruments. On a well-proportioned round brilliant of 57 facets, the table sits between 53 and 58 per cent of the diameter and the total depth between 59 and 62.5 per cent. Outside those ranges light leaks away through the bottom and the stone goes dull. Our gemmologists measure this; they do not estimate it.

Colour and clarity are gradings on a scale, and that stays human work. It is exactly there that a laboratory report does most of its work: if a buyer has to decide for himself whether he is looking at a G or a J, he will price that doubt in his own favour. With carat and cut there is little to doubt; with the other two there is.

The shape helps decide who the buyer is

The round brilliant is the market standard and therefore has the broadest demand. Fancy shapes such as princess, emerald, oval and cushion follow fashion, and each shape has its own audience.

An emerald cut is the most unforgiving of the stone itself. Its open step facets hide nothing: inclusions and a tint of colour show up faster than on a brilliant, where the facet work sparkles a great deal away. The same grading on an emerald therefore does not mean the same as on a brilliant, and we account for that shape by shape.

An old cut does not take the modern yardstick

Diamonds from before roughly 1940 were cut by hand: a smaller table, a high crown, often an open culet. By modern cut standards they score poorly — but that is the wrong yardstick. Collectors and jewellers seek out old European and old mine cuts precisely for that warm, soft light.

We therefore assess such a stone twice: as an antique stone in its own market, and as a candidate for recutting into a brilliant. The higher of those two outcomes sets our offer. So do not have an heirloom recut in advance: the weight loss is heavy, and the antique market often pays more for the original than the modern market pays for the result.

About what diamond is doing in the trade there is no figure here — we measure precious metal rates, not stones. What we do measure is the setting, and many diamond rings are platinum: on 27 August platinum stood at 51.07 euros per gram, 12.7 per cent above the 45.33 euros of 28 July. That part of your ring has a rate. The stone does not.