Cut: the one that makes it sparkle
Cut is how well the stone is shaped and angled, and it is what makes a diamond look alive from across the room. A well-cut smaller stone will out-sparkle a bigger, badly cut one every time.
This is the grade you do not compromise on. For round diamonds, stick to Excellent (or Ideal, depending on the lab's wording) and move on with your life.
Color: near-colorless is the sweet spot
Color grades run from D (icy white) down the alphabet toward visibly warm. Here is the thing: side by side under lab lights you can tell a D from an H. On a hand, in a restaurant, you cannot.
- G to H looks white to basically everyone and costs far less than D to F.
- Setting matters. In yellow or rose gold you can drop another grade or two, because the metal warms everything anyway.
Clarity: buy eye-clean, not flawless
Clarity measures tiny internal flaws. The magic phrase is "eye-clean": no flaws visible to a normal human at normal distance. Nobody proposes with a loupe.
- VS1 or VS2 is almost always eye-clean. That is the safe zone.
- SI1 can be a bargain if you can inspect real photos or video of the exact stone first.
- Flawless is for collectors and people who enjoy overpaying.
Carat: buy just under the round number
Prices jump at the round numbers because demand does. A 0.90 carat stone looks nearly identical to a 1.00 carat on a finger, and costs noticeably less. Same trick works at 1.90 vs 2.00.
Jewelers call these "shy" weights. Buy shy, pocket the difference, upgrade the setting or the honeymoon.
Excellent cut, G-H color, VS clarity, shy carat weight. That combination is what a smart buyer orders.