Most underrated. Hardest to fake.
Cut
Cut is the only "C" that's actually about how the diamond was made — the angles, polish, and proportions of the cuts on the stone. Color, clarity, and carat are about the rough material. Cut is the craftsmanship.
A poorly cut D/IF stone looks dull. A perfectly cut H/SI1 stone looks like it's lit from inside. GIA grades cut Excellent / Very Good / Good / Fair / Poor on round brilliants only — fancy shapes don't get a cut grade, only polish and symmetry. That's why fancy shapes price 15-25% lower than rounds at the same color and clarity.
Buy rule: Excellent or Very Good cut. Below that the stone looks dead and resells worse.
Most overrated by buyers. Priced super-linearly.
Carat
Carat is weight, not size. A 1.0ct round is about 6.5mm across the top; a 2.0ct is about 8.1mm — bigger, but not double. Yet a 2ct natural G/VS2 retails for ~$12,000 and a 1ct retails for ~$5,000. You don't pay double — you pay 2.5×.
The reason: large rough is exponentially rarer than small rough. A diamond cutter recovers about 50% of the rough weight when polishing, so producing a 2ct polished stone requires 4ct of clean rough. That kind of rough comes out of the ground much less often.
Save rule: Buy just under the popular size thresholds. A 0.95ct stone looks identical to 1.00ct face-up but costs 15-20% less. Same for 1.45ct vs 1.50ct, 1.95 vs 2.00ct.
Visible past J. Below H, you can usually save money.
Color
GIA grades diamond color D (colorless) through Z (light yellow). D-F are colorless, G-J are near-colorless, K and below show visible warmth. Most non-jewelers can't tell D from G when the stone is in a setting.
Yellow gold settings hide warmth. Platinum and white gold reveal it. So a G in white gold looks identical to a D in white gold; an H in yellow gold looks identical to an E in yellow gold.
Save rule: G-H in white metal, I-J in yellow metal. You'll never notice the difference, and you save 10-25% over D-F.
Sounds important. Mostly invisible without a loupe.
Clarity
Clarity grades the inclusions inside the stone, viewed at 10× magnification. FL/IF (no inclusions visible at 10×) are the rarest. VS1/VS2 inclusions are invisible to the naked eye. SI1/SI2 inclusions are sometimes visible if you know exactly where to look. I1/I2/I3 inclusions are visible to anyone.
The clarity-grade increments are bigger than they look. FL → IF is essentially nothing in real-world appearance but adds 20% to the price. VS2 → SI1 is also nothing visible to the naked eye but saves 15-20%. The cliff is at SI2 → I1, where inclusions start being visible.
Save rule: "Eye-clean SI1" or VS2. Below that you start to see things; above that you're paying for invisible-to-everyone perfection.
If you have to compromise on one C, compromise on clarity.
Cut is the only C that affects how the diamond actually looks at arm's length. Color and clarity affect how it looks under a loupe. Carat affects price more than appearance. The cheapest beautiful diamond is an Excellent-cut, G-H, SI1 — and it'll look identical to the D/FL stone next to it on the same finger.