In the gemstone and diamond trades, carat-up (also encountered as size up) refers to the deliberate practice of cutting, selecting, or sorting stones so that they meet or exceed a weight threshold at which per-carat prices rise disproportionately. These thresholds — commonly called magic sizes or magic carats — represent psychological and commercial breakpoints in the market: a stone weighing 1.01 ct may command a meaningfully higher price per carat than one at 0.98 ct, even though the visible difference in physical size is negligible. The practice is widespread across both the diamond and coloured-stone sectors, and its influence on cutting decisions is considerable.

The Logic of Magic Sizes

Retail and wholesale pricing schedules — Rapaport in the diamond trade, and analogous per-carat grids used by dealers in ruby, sapphire, and emerald — are typically structured in weight brackets. Once a stone crosses a bracket boundary, the applicable per-carat rate for the entire stone jumps to the higher tier. The most commercially significant thresholds are broadly recognised as 0.50 ct, 1.00 ct, 2.00 ct, 3.00 ct, 5.00 ct, and 10.00 ct, with 1.00 ct and 5.00 ct representing particularly sharp inflection points in many markets. A fine unheated Burma sapphire, for instance, that can be delivered at just over 5.00 ct may achieve a per-carat price substantially above what the same material would fetch at 4.85 ct, even with equivalent colour and clarity.

The premium reflects genuine market demand: buyers — whether retailers, collectors, or end consumers — attach symbolic and practical value to round-number weights. A one-carat solitaire ring is a distinct retail category; a 0.93 ct stone, however well cut, does not satisfy that brief in the same way.

Cutting Consequences

The economic incentive to carat-up can place a lapidary in direct conflict with the pursuit of optimal proportions. To preserve weight above a magic-size threshold, a cutter may accept a slightly steeper pavilion, a thicker girdle, or a shallower crown angle than ideal optics would dictate. In the coloured-stone trade, where cutting standards are already more variable than in the diamond sector, this tension is particularly visible. A well-proportioned 0.95 ct sapphire that would display superior brilliance and colour saturation may be left at 1.02 ct with a bulging pavilion, because the weight premium outweighs the optical penalty in the dealer's calculation.

Conversely, a stone that cannot reach the next threshold without unacceptable sacrifice of yield or quality may be cut down from a borderline weight — for example, from 1.03 ct to 0.98 ct — to achieve better proportions while accepting the lower bracket. This inverse decision is sometimes called cutting away from the size.

Parcel Sorting

The term carat-up is also applied to the sorting of melee and parcel goods. A dealer offering a parcel described as "carat-up" is indicating that every stone in the lot meets or exceeds a specified minimum weight — commonly 1.00 ct — and has been culled accordingly. This usage is standard in wholesale communications and auction-house parcel descriptions, where it functions as a shorthand quality-of-selection indicator rather than a cutting instruction.

Market and Valuation Implications

For appraisers and buyers, awareness of carat-up practice is essential to accurate valuation. A stone sitting just above a magic-size threshold warrants scrutiny of its proportions: if the depth percentage or girdle thickness is outside normal ranges, the possibility that weight was retained at the expense of cut quality should be considered. Gemmological laboratories including the GIA note weight in their reports to two decimal places, and experienced valuers cross-reference reported weight against measured dimensions to assess whether a stone's proportions are consistent with its carat weight.

It is equally important to recognise that the per-carat premium attached to magic sizes is not permanent or universal. Market conditions, stone quality, and origin premiums can all modulate or override the threshold effect. In the finest unheated rubies and sapphires of exceptional colour, the origin and treatment status may dominate pricing to a degree that renders the magic-size premium secondary.

Further Reading

• GIA Gems & Gemology — Coloured Stone Pricing Fundamentals
• GIA Gem Encyclopedia