Published on19/07/2026
If you plot diamond price against weight, you do not get a smooth upward line. You get a staircase: price runs flat for a stretch, jumps at a mark, then runs flat again.
Those marks are the round numbers the trade has names for: 0.30, 0.50, 0.70, 0.90, 1.00, 1.50 and 2.00 carat. The one-carat mark is the tallest step, and the one where the gap is most dramatic.
The cause is entirely buyer psychology, not physics or geology. Everyone wants to say a one-carat stone. Nobody wants to say a ninety-nine pointer. Buyers hunt for round numbers, so demand piles up immediately above each mark; sellers know this, so cutters also fight to retain weight to reach a mark, even when doing so means sacrificing the proportions.
That last point matters and few people know it: some stones that come in at exactly 1.00ct were cut DEEPER than optimal purely to hold enough weight to hit the mark. The result is a one-carat stone with the table diameter of a well-cut 0.92ct, and less fire besides. You pay extra for the round number and receive a stone that looks smaller and darker.
Look at the actual numbers. A well-cut 1.00ct round brilliant measures about 6.5 mm across. A 0.90ct measures about 6.3 mm. A difference of 0.2 mm — thinner than two sheets of paper. Side by side on a white tray under close inspection you might spot it; on a hand, never.
The price is another matter entirely. Depending on the market and the other Cs, a 0.90ct typically costs 15 to 25 percent less than a 1.00ct. On a ring around 100 million dong that is roughly 20 million — enough to upgrade the Cut from Very Good to Excellent, or to move up to a distinctly better-made setting.
Applying this is simple: aim for the band JUST BELOW a mark. Instead of 1.00ct, look at 0.90 to 0.96ct. Instead of 0.50ct, look at 0.45 to 0.48ct. Instead of 2.00ct, look at 1.90 to 1.95ct. Say those numbers to the seller directly; it is an entirely normal request in the trade and they will immediately understand you know what you are doing.
One practical note: stones just below a mark are scarcer, because cutters avoid producing them. Be prepared to wait a little longer, or ask the seller to search their network rather than choosing only from the case.
And one warning. Do not let mark-hunting drag you down into badly proportioned stones. A 0.92ct cut Excellent is a bargain. A 0.92ct cut Fair is not a bargain; it is simply a poor stone that costs less. The carat mark is a place to save money, not a place to lower your standards.
Finally, if what you actually want is a ring that LOOKS large, the carat mark is not the strongest lever. A 0.7ct in a halo setting reads larger than a 1.0ct alone, at considerably lower cost. Shape works the same way: oval, marquise and pear cuts present more table area than a round at the same weight, because they are elongated along one axis.