Carat
How diamond weight is measured, why it's not the same as visual size, and how weight thresholds shape pricing in the trade.
Carat is the unit the diamond trade uses to measure weight. One carat equals exactly 0.200 grams, or 200 milligrams — a standard fixed by the International Committee on Weights and Measures in 1907 and used by every laboratory and grading report today. The word itself traces back to the carob seed, which the ancient trade used as an informal reference weight because ripe seeds were remarkably uniform in mass.
For stones under one carat, weight is commonly quoted in points, where one carat equals one hundred points. A 0.75 ct stone is therefore a seventy-five pointer.
Weight does not scale with size
The most common misconception — from buyers new to the trade and from end consumers alike — is that a diamond twice the weight is twice the size. It isn't.
A round brilliant's face-up diameter scales with the cube root of its weight. A well-proportioned one-carat round measures roughly 6.5 mm across the girdle. A two-carat round of the same quality measures closer to 8.2 mm — about 26% wider, nowhere near double. The ratio is geometric: double the weight spreads across all three dimensions of a three-dimensional object.
Weight ratio: 2.00×· Diameter ratio: 1.26×
Diameters use the industry approximation d ≈ 6.5 × ∛(ct) for a well-proportioned round brilliant. Actual cut geometry can shift face-up diameter by a few tenths of a millimetre.
Use the comparator above to feel the relationship. Pick the preset 0.9 vs 1.0 ct and the two stones are visually indistinguishable — the diameter differs by ~0.2 mm, a gap no trained eye picks up without a loupe and a calliper. Pick 1.0 vs 2.0 ct and the visual jump is meaningful but much less dramatic than the number suggests. The cube-root relationship is why a 3 ct stone feels less than "three times bigger" than a 1 ct stone in a setting.
How carat is reported
Gemological laboratories report weight to two decimal places and round to the nearest hundredth. A stone that physically weighs 0.997 ct may be reported as 1.00 ct; a stone weighing 0.993 ct is reported as 0.99. The difference is a hundredth of a point, but in trading terms it crosses a psychological boundary that dealers and price sheets treat as a hard discontinuity.
For very small stones — melee goods under 0.20 ct — weight is typically reported in thousandths and traded by parcel rather than by individual certificate.
Why weight drives price non-linearly
If weight alone determined price, the cost per carat would be roughly constant. In practice, price per carat rises sharply at specific weight thresholds because rough yielding a clean, well-proportioned finished stone at those weights is disproportionately rare.
The thresholds the market watches most closely:
| Typical ppct step-up | Why the market pays for it | |
|---|---|---|
| 0.50 ct | +10–15% | Half-carat psychological boundary and common retail spec |
| 1.00 ct | +15–25% | Largest single jump in the sub-2 ct range; strongest retail demand |
| 1.50 ct | +8–12% | Clean, well-proportioned rough at this weight is disproportionately rare |
| 2.00 ct | +15–20% | Investment-grade threshold; liquidity and resale profile shift |
| 3.00 ct | ppct ~2× a 1 ct equivalent | Rough yield drops sharply; premium accelerates non-linearly |
| 5.00 ct+ | Individually negotiated | Each stone trades on its own merits rather than a band |
This is why experienced buyers pay close attention to weight band. A 0.95 ct stone sits just below the 1 ct psychological line, so its per-carat price reflects the sub-one-carat band. A 1.00 ct stone of identical quality prices against the one-carat-plus band — the same physical stone size, a materially different price because of where the decimal lands.
Carat in context: what weight does not tell you
Weight on its own is a poor proxy for value. Two stones at 1.00 ct can differ by a factor of three in price per carat depending on their other characteristics:
- A well-cut 1 ct round returns more light and faces up larger than a deep-cut or shallow-cut stone of the same weight. Proportions matter more than a tenth-point of weight
- A higher color grade (D–F) at the same weight commands a substantial premium over a tinted stone (J–K)
- A higher clarity grade (VVS and above) prices above eye-clean SI goods at the same weight
- Fluorescence, certification source, and shape further modulate the per-carat number
In short: carat is the axis against which every other characteristic is priced. Two stones of the same weight tell you nothing until you know the rest of the grading report.
Practical notes for the trade
- Watch the rounding. When sourcing against a buyer's brief for "1 ct", ask whether they will accept 0.96–0.99 at the correspondingly lower per-carat price, or whether they need the certificate to read 1.00+. The answer often unlocks better value on otherwise identical goods
- Shape affects apparent size at the same weight. An oval or marquise of a given weight looks larger face-up than a round of the same weight because more of its mass sits in the crown rather than the pavilion. Buyers sensitive to visible size at a fixed budget can swap shape for a perceived size step up
- Spread goods. A shallowly cut stone (a "spread make") reads as a larger face-up for its weight but sacrifices brilliance. Pricing models penalise this, but it remains relevant for goods destined for settings where face-up size dominates the sale