A 0.25 carat round diamond is about 4.05 mm across. The same weight is 4.9 × 3.6 mm as an oval, 3.5 mm as a princess, 4.4 × 3.1 mm as an emerald cut and 6.6 × 3.3 mm as a marquise. Each cut is drawn at true 1:1 size on a finger below.
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| Actions | |||||||
|---|---|---|---|---|---|---|---|
| 4.05 | 1 : 1 | 2.49 | 12.9 | 25% | 21% | ||
| 4.91 × 3.64 | 1.35 : 1 | 2.26 | 14.0 | 22% | 19% | ||
| 4.38 × 3.13 | 1.4 : 1 | 2.03 | 13.2 | 19% | 16% | ||
| 5.58 × 3.49 | 1.6 : 1 | 2.16 | 13.7 | 21% | 18% | ||
| 3.85 × 3.50 | 1.1 : 1 | 2.28 | 12.7 | 21% | 18% | ||
| 3.50 | 1 : 1 | 2.45 | 12.3 | 21% | 18% | ||
| 6.60 × 3.30 | 2 : 1 | 1.98 | 15.2 | 20% | 17% | ||
| 4.16 × 3.33 | 1.25 : 1 | 2.20 | 13.5 | 20% | 18% |
| Actions | ||||
|---|---|---|---|---|
| 3.52 | 4.05 mm | 0.25 ct | ||
| 3.21 | 4.18 mm | 0.23 ct | ||
| 4.00 | 3.89 mm | 0.28 ct | ||
| 2.72 | 4.42 mm | 0.19 ct |
05 / Field guide
What 0.25 carat looks like in millimetres: every cut at standard proportions, on fingers of different sizes, against the weights either side and in other stones. Sizes are estimates from the trade formulas; a real stone varies with its cut.
0.25 carat is 50 mg. As a round brilliant at a 61.5% depth it is 4.05 mm across and about 2.49 mm deep. marquise (6.6 × 3.3 mm), oval (4.9 × 3.6 mm), pear (5.6 × 3.5 mm) spread the same weight over the largest outlines; the princess (3.5 mm) faces up smallest because more of its weight sits below the girdle.
Face-up, the marquise covers 15.2 mm² against 12.3 mm² for the princess, 24% more for the same 0.25 carats.
A 4.05 mm round covers about 26% of the width of a US 5 finger (15.7 mm), 23% of a US 7 and 21% of a US 10. It sits well inside the finger’s width, a delicate look. Set the finger chips to your own size to see it drawn at 1:1.
A 0.5 carat round is 5.11 mm, only 26% wider, because weight grows with the cube of the size.
Density changes the size of a stone of the same weight. 0.25 ct of moissanite (SG 3.21) is about 4.18 mm round, sapphire or ruby (SG 4.00) 3.89 mm and emerald (SG 2.72) 4.42 mm, against 4.05 mm for diamond. Moissanite is usually sold by diamond equivalent weight, so a “0.25 ct DEW” moissanite is cut to the 4.05 mm diamond size and weighs about 0.23 ct.
Questions
About 4.05 mm across as a round brilliant. As an oval it is about 4.9 × 3.6 mm, a princess 3.5 mm, an emerald cut 4.4 × 3.1 mm and a cushion 3.9 × 3.5 mm.
The marquise, at 6.6 × 3.3 mm with 15.2 mm² face-up. The princess looks smallest at 3.5 mm.
Sold by diamond equivalent weight, it is the diamond size, 4.05 mm round, and weighs about 0.23 ct. A moissanite weighing a true 0.25 ct is about 4.18 mm.
A 4.05 mm round covers about 25% of the 16.5 mm width of a US 6 finger.
As rounds, 5.11 mm against 4.05 mm: about 26% wider for 100% more weight.