Lesson 5 · Diamonds explained · 43 seconds
Same carat weight. Different look.
Carat is a weight, not a size. One carat is 200 milligrams. Two diamonds can weigh exactly the same and still measure differently across the top, because a deeper cut carries more weight underneath where you cannot see it.
What this lesson shows
Carat is a weight. The face-up size depends on the cut.
Two modeled round-brilliant diamonds in the lesson both weigh 1.00 ct exactly. One is cut deeper, so it measures 5.9 mm across the top. The shallower one measures 6.4 mm. Both are "one carat" on paper.
What this means for shopping: compare the millimeter measurements and the cut grade, not just the carat number.
Three quick takeaways
What to actually look at on a listing
- Read the millimeter measurements (face-up diameter) alongside the carat weight. Those are the dimensions a viewer actually sees.
- Check the cut grade on the grading report. A well-cut stone tends to have proportions that place its weight where you can see it; a poorly cut stone can hide weight below the girdle.
- Two stones at the same carat weight can still differ in appearance because of cut, proportion, clarity and color. Compare dimensions and the full grading report, not carat alone.
The stones in the lesson are computed models scaled by exact volume at 3.52 g/cm3 (diamond's density, per FTC 16 CFR 23.12) to equal 1.00 ct. They are not graded stones and carry no color or clarity claim.
Read the transcript
Same carat weight. So why does one diamond look bigger? Carat is a weight, not a size. One carat is two hundred milligrams, a fifth of a gram. These two modeled stones weigh exactly the same. Now turn them to the side. This one is cut deeper, so more of its weight sits underneath, where you can't see it from the top. Face up, it measures smaller across. So compare the millimeter measurements and the cut, not just the carat number. Both are on a grading report. Save this before you compare diamonds.
Sources
- GIA 4Cs, "Diamond carat weight": one carat equals 200 milligrams. 4cs.gia.edu
- GIA 4Cs, "Nine things about diamond carat weight you need to know": weight can hide in or below the girdle; do not estimate size from carat alone. 4cs.gia.edu
- FTC 16 CFR 23.12 (Guides for the Jewelry Industry): defines diamond with hardness 10 and specific gravity approximately 3.52. law.cornell.edu
Related lessons
See also lab-grown diamond vs cubic zirconia for how to confirm you are actually comparing diamond to diamond, and total carat weight explained for how to read ct tw on earring and three-stone listings.
Next step
Shop lab-grown diamond rings.
Our lab-grown diamond engagement rings and jewelry list carat weight, millimeter measurement and cut, so you can compare on the dimensions that matter.