Diamonds Over 5cts

There is a point, somewhere around five carats, where the ordinary rules of pricing begin to fall away. A weekly list works well for the great body of the trade — the half-carats and the one- and two-carat stones that move in volume. The largest diamonds do not move in volume. They are priced as individuals, and the list can only gesture at where their value begins.

Why the list lets go

The reason is rarity, and it compounds. The rough capable of yielding a clean five-carat polished stone is scarce; the rough capable of yielding ten or twenty carats of fine colour and clarity is scarcer still, by a margin that grows faster than the weight itself. Price-per-carat does not rise in a straight line as stones get larger — it climbs steeply, because so few crystals of that size and quality ever reach the surface, and fewer still survive the journey to the cutter intact.

A list assumes a deep, liquid market of comparable goods to average across. At the top of the weight scale that market thins to almost nothing. Each great stone is close to unique, and a number averaged from its near-absent peers means little.

What sets the price instead

For these diamonds, value is settled less by formula than by occasion. The decisive characteristics are the familiar ones — exceptional colour, high clarity, a cut that does the weight justice — but their effect is magnified, and to them is added the simple fact of size, which carries its own prestige. Provenance can matter too: a name, a history, a place in the record.

The truest test of value is the saleroom. The largest and finest stones are sold at auction, where a small circle of collectors and houses compete, and where the great diamonds set the records that the rest of the trade reads as a marker. A price arrived at there is not read off a list — it is discovered in the room, between the few people prepared to own such a thing.