Price Per Carat: Why Size Multiplies Value

Price per carat is not a constant. It climbs with size, steps at thresholds, and steepens with quality — which is why buying weight is the common mistake.

The arithmetic that catches people is simple enough. A one-carat fine ruby at a given price does not imply a five-carat stone at five times that price. The five-carat stone will cost a multiple of the multiple, and in fine colour the gap is very large.

Why the curve bends

Large rough with fine colour is not rarer in proportion to its size. It is rarer than that. Crystals grow with flaws, and the probability that a given volume is clean, well-coloured and free of fractures falls away sharply as the volume rises. Then cutting takes its share, and the proportional loss is worse on large rough because there is more geometry to work around.

The trade has understood this for a long time. Tavernier, writing in the seventeenth century, expressed the relationship as weight squared against a base price — value rising with the square of the carat weight. Nobody prices that way today, but the shape of the curve he described is still the shape.

The thresholds

The curve is not smooth. Prices step at weights the market has decided to care about — one carat, three, five, ten — and the step can be sharp. A stone at 0.98 carats and a stone at 1.02 carats are the same stone to a gemmologist and are not remotely the same stone to a price list.

That has a consequence you can see in the goods. Cutters hold weight above a threshold at the cost of proportion, which is where deep pavilions and heavy girdles come from. You are sometimes offered a stone whose only reason for being that shape is a number — and what that costs the stone optically is rarely reflected in the asking price.

Quality steepens it

The curve is steepest at the top of the quality range and nearly flat at the bottom. Commercial material does not reward size much, because commercial material is not scarce at any size. Fine material rewards it enormously.

Which produces the mistake I see most often. Two stones of identical weight can differ by an order of magnitude on colour alone. Weight is the easiest variable to understand and the least useful to optimise. Colour first; then clarity, in the sense of what is actually visible face-up; then cut; then weight, and weight only as the last tie-breaker.

Nobody has ever admired a stone for its weight. Buy the colour, and let the carats be whatever they turn out to be.