Ask a diamond buyer what a stone costs and they think in price per carat. Ask a coloured stone dealer the same question and the answer comes with a caveat: price per carat is not a fixed number, it climbs as the stone gets bigger. A 5-carat sapphire is rarely twice the price of a 2.5-carat sapphire of identical colour, clarity and origin — it is often three to five times the price, and sometimes far more. This article explains why that curve exists, where the thresholds actually sit, and how buyers and investors should factor size into a purchase decision.
Rarity, Not Volume, Drives the Price Curve
Corundum forms in commercially significant quantities in a handful of geological settings, but large, clean, well-coloured crystals are a vanishingly small fraction of what comes out of the ground. Most rough is small, included, or poorly coloured. By the time a parcel has been sorted, cut and certified, the population of fine stones thins out sharply as weight increases — and because cutting a gem removes 50-70% of the rough weight, producing a finished 5-carat stone requires rough that is exceptionally rare to begin with.
The result is a supply curve that falls away faster than demand does. Buyers who want a 5-carat unheated Ceylon sapphire in a fine royal blue are competing for a very short list of available stones, and that scarcity is priced in. This is why the trade quotes price per carat in size brackets rather than as a single figure: a dealer's sheet will show a different per-carat range for 1-2ct, 2-3ct, 3-5ct, and 5ct-plus, with each bracket stepping up materially.
Where the Thresholds Actually Sit
The steps are not evenly spaced, and they matter more at some points than others. In practice, the commercially significant breakpoints for sapphire and ruby tend to cluster around:
- 1 carat — the psychological floor for a centre stone. Below it, stones are largely priced as accent or calibrated goods.
- 2 carats — the first meaningful step for engagement-ring buyers; supply is still reasonable, so the jump is moderate.
- 3 carats — a genuine narrowing. Fine material in this range is noticeably harder to source, particularly unheated.
- 5 carats — where the curve turns steep. Stones above 5ct in top colour move into collector and investment territory.
- 10 carats — effectively a different market. At this size, individual stones are traded on their own merits rather than against a price sheet.
These thresholds also explain a quirk that surprises new buyers: a 2.95-carat stone and a 3.05-carat stone of the same quality can differ in price by more than 10%, because the heavier one clears a bracket. Sellers know this, which is why cutters will sometimes preserve weight at the cost of ideal proportions to land a stone above a threshold — a trade-off worth checking for, since a bulged or windowed stone bought for its weight is a poor value.
Why the Curve Is Steeper for Ruby
Ruby is scarcer than sapphire in large sizes, and the price curve reflects it. Blue sapphire is produced in commercial volume from Sri Lanka, Madagascar, Australia and elsewhere, and clean stones above 5 carats, while uncommon, do surface regularly. Fine ruby above 3 carats — with genuinely saturated red colour and no glass filling — is a materially thinner market, and the Burmese material that commands the highest premiums has been supply-constrained for decades.
The practical consequence for buyers is that stepping up one carat in ruby costs proportionally more than the same step in sapphire. Many jewellery designers work around this by using well-matched smaller stones or a strong 1-2ct centre rather than chasing size. Our ruby solitaires are sorted by weight bracket for exactly this reason — it lets buyers see where the price steps fall before committing to a size.
When Bigger Is Not Better
Size amplifies quality, in both directions. A larger stone shows its colour, its cut and its inclusions more plainly, so a mediocre 5-carat sapphire looks worse than a mediocre 1-carat one and is harder to resell. The stones that hold value over time are the ones where size and quality move together: good saturation, an eye-clean appearance, a cut that returns light across the whole face, and credible lab documentation of any treatment.
For buyers weighing budget against size, the more reliable approach is usually to fix the quality floor first — colour and cut you are genuinely happy with — and then buy the largest stone that fits. A well-cut, unheated 2.5-carat sapphire in fine colour is a stronger asset than a heated, windowed 5-carat stone bought purely for its weight. That logic holds especially in the unheated segment, where the premium is already substantial; browse our unheated sapphires to see how per-carat pricing shifts across size brackets when treatment is held constant.
What This Means at the Buying Table
Three practical takeaways. First, always ask for price per carat, not just total price — it is the only way to compare stones of different weights and to see whether you are being quoted inside or outside the normal bracket. Second, treat a stone sitting just over a threshold with mild scepticism and check the cut. Third, if you are buying for long-term value, remember that the steepness of the curve cuts both ways: it is the reason large fine stones appreciate, and also the reason a large mediocre stone is difficult to move.
At Thai Gems, we have been cutting and trading corundum out of Bangkok for over 70 years, and we price transparently by size bracket with treatment and origin disclosed on every stone. Explore our full range of sapphires and rubies, or contact us for trade pricing on specific sizes and calibrated parcels.