Every diamond that matters comes with a report card: cut, color, clarity, carat. The industry presents these four as equals, which is the first polite fiction to discard. They are not equals. One of them does nearly all the work.
Cut is the engine
Cut describes how well the stone's angles return light to your eye, and it is the entire reason a diamond does the thing diamonds do. A superbly cut modest stone will embarrass a large dull one in any lighting a restaurant can offer. Hold the line at the top cut grade for round stones and treat it as the one non-negotiable on the whole report.
Color and clarity are thresholds, not ladders
Color runs from icy D down the alphabet into visible warmth. The useful truth is that the middle of the near-colorless range looks white to every human who is not holding a comparison stone under laboratory light. Clarity works the same way: the grade to buy is the one where flaws stop being visible to the naked eye, often called eye-clean, and every grade above that threshold is money spent on a secret.
You are not buying a report card. You are buying what a person can see from a dinner table away.
Carat is where the games are played
Demand piles up at round numbers, so prices leap at them. A stone a few hundredths shy of the milestone looks identical on a hand and costs meaningfully less. Jewelers know these as shy weights. Buy shy and let someone else fund the round number's vanity premium.
The short of it: insist on the best cut, buy color and clarity at the threshold where differences stop being visible, and buy carat just under the round numbers.