Ruby and sapphire carat size chart: how big is a 1 carat ruby in mm?
A 1 carat ruby is about 6.1 mm across as a round, smaller than a 1 carat diamond (6.5 mm), because , the mineral of ruby and sapphire, is denser (specific gravity 4.00 against 3.52). A 1 carat oval is about 7.3 x 5.5 mm and a square cushion about 5.8 mm. Sapphire matches; deeper cuts look smaller.
How big is a 1 carat ruby?
About 6.1 mm across as a round, 7.3 x 5.5 mm as an oval and 5.8 mm as a square cushion, worked out by the method below. A 1 carat round diamond is 6.5 mm: GIA’s research journal gives a 6.50 mm round brilliant diamond as about 1.00 carat. So a ruby looks a little smaller than a diamond of the same weight, though only by about 4 to 6% across, depending on the cut. A 5 carat ruby comes out at about 10.4 mm as a round or 12.4 x 9.3 mm as an oval; how big is a 5 carat ruby runs the other way, from millimetres to carats, and the two pages agree to within a few tenths of a carat. Shape matters when you shop: GIA notes that rubies are commonly cut into oval and cushion shapes, and that rounds, emerald cuts and other shapes are rare in larger sizes and higher qualities. Lab rubies, though, are sold as big rounds too, as big lab gems shows.
What size is each carat weight in millimetres?
Face-up sizes for ruby and sapphire cut 55% as deep as they are wide, rounded to 0.1 mm. The oval is 4 to 3 in length to width, the shape of an 8 x 6 mm stone, and the cushion is square. The last column is a round diamond of the same weight, for comparison. Treat every figure as approximate: the same weight can face up a few per cent bigger or smaller depending on the cut.
| Carat weight | Round ruby or sapphire (mm) | Oval, 4:3 (mm) | Cushion, square (mm) | Round diamond, same weight (mm) |
|---|---|---|---|---|
| 0.5 ct | 4.8 | 5.8 x 4.3 | 4.6 | 5.2 |
| 0.75 ct | 5.5 | 6.6 x 5.0 | 5.3 | 5.9 |
| 1 ct | 6.1 | 7.3 x 5.5 | 5.8 | 6.5 |
| 1.5 ct | 7.0 | 8.3 x 6.2 | 6.6 | 7.4 |
| 2 ct | 7.7 | 9.2 x 6.9 | 7.3 | 8.2 |
| 3 ct | 8.8 | 10.5 x 7.9 | 8.3 | 9.4 |
| 4 ct | 9.7 | 11.5 x 8.7 | 9.2 | 10.3 |
| 5 ct | 10.4 | 12.4 x 9.3 | 9.9 | 11.1 |
| 7 ct | 11.7 | 13.9 x 10.4 | 11.1 | 12.4 |
| 10 ct | 13.1 | 15.7 x 11.8 | 12.5 | 14.0 |
| 15 ct | 15.1 | 17.9 x 13.5 | 14.3 | 16.0 |
| 20 ct | 16.6 | 19.7 x 14.8 | 15.7 | 17.6 |
How is the ruby size chart worked out?
Carat is a weight, not a size: one carat is 0.20 gram, as GIA puts it. Turning weight into millimetres takes the stone’s density and its shape. We used a formula for estimating the weight of a mounted coloured stone, published by Charles Carmona in GIA’s journal Gems & Gemology in 1998 and built from more than 5,000 calibrated amethysts and citrines plus dozens of synthetic sapphires: carats = length x width x depth (in mm) x specific gravity x shape factor. His shape factors are 0.00200 for a round, 0.00210 for an average oval and 0.00235 for an average square cushion. GIA gives ruby and sapphire a specific gravity of 4.00, and the Handbook of Mineralogy gives measured values for corundum of 3.98 to 4.10. We assumed each stone is 55% as deep as it is wide, solved for size, and checked every row back to its weight with a script. That depth gives about 0.95 carat for a 6 mm round and 2.25 carats for an 8 mm round, close to the figures on our 5 carat ruby page. Many coloured stones are cut heavier than that, as the last section explains, and a heavier-cut stone of the same weight is smaller across. For a round ruby the formula boils down to diameter x diameter x depth x 0.008. As a cross-check, a round ruby cut to exactly a diamond’s proportions would be 6.5 mm times the cube root of (carats x 3.52 / 4.00): 6.2 mm at 1 carat, against 6.1 mm in the chart.
Why is a ruby smaller than a diamond of the same carat weight?
Because ruby is heavier for its size. GIA gives corundum a specific gravity of 4.00 and diamond 3.52, so a carat of ruby takes up 12% less space than a carat of diamond. Spread over three dimensions, a ruby of the same weight and proportions is about 4% smaller across: 6.2 mm against 6.5 mm at 1 carat, and 10.7 mm against 11.1 mm at 5 carats. Sapphire is corundum too, so a sapphire and a ruby of the same weight and cut are the same size. Moissanite is the opposite case: at a specific gravity of 3.22 it is lighter than diamond, and a 1997 study in GIA’s journal worked out that a moissanite the same diameter and cut as a 6.50 mm, 1.00 carat diamond weighs about 0.91 carat. Its sizes are on the moissanite carat size chart.
Why can two 1 carat rubies look different sizes?
Because some of a stone’s weight sits where you cannot see it. GIA makes the point for diamonds: extra weight can be hidden in or below the girdle, and a poorly cut 1.20 carat diamond may look the same size as a well-cut 1.00 carat one. Coloured stones vary more. Carmona notes that they are usually cut with steeper crowns, bigger pavilion bulges and thicker girdles than diamonds, all of which add weight, and GIA says ruby rough is so expensive that many cutters try to keep as much weight as possible. To save weight they may also cut flat rough into shallow stones, which can leave a window, a see-through area in the stone. In Carmona’s worked examples, a sapphire he calls very shallow is 52% as deep as it is wide, and an emerald with a deep pavilion is 78%. At those two depths a 1 carat round ruby comes out at about 6.2 mm and 5.4 mm. Carmona quoted the older formulas’ own warning that even for an experienced grader they are accurate only to within 10 to 15%, and wrote that his formulas and methods could cut that margin of error to 5% or less, with experience. So buy by millimetres: if a listing gives both size and weight, check them against the chart, and if the weight is far off for the size, ask the seller before you buy. For lab rubies sold by size, see the large lab rubies guide.
Frequently Asked Questions
How big is a 2 carat ruby?
About 7.7 mm across as a round, 9.2 x 6.9 mm as an oval or 7.3 mm as a square cushion, cut 55% as deep as it is wide. A 2 carat round diamond is about 8.2 mm.
Is a 1 carat sapphire the same size as a 1 carat ruby?
Yes, if they are cut alike. Both are corundum, which GIA gives a specific gravity of 4.00, so the same chart works for sapphire of any colour and for ruby: about 6.1 mm as a round at 1 carat.
How do you convert ruby millimetres to carats?
For a round, multiply diameter x diameter x depth, all in millimetres, by 0.008. An 8 mm round that is 4.4 mm deep comes to about 2.25 carats. For an oval use length x width x depth x 0.0084, and for a cushion x 0.0094. The results are estimates: the formulas were built from well-proportioned stones.
How big is a 5 carat ruby?
About 10.4 mm across as a round, 12.4 x 9.3 mm as an oval or 9.9 mm as a square cushion, cut 55% as deep as it is wide. A deeply cut round can be under 10 mm; a 5 carat round diamond is about 11.1 mm.
Sources
- Carmona, Gems & Gemology, Fall 1998: Estimating weights of mounted colored gemstones (carats = length x width x depth x SG x shape factor; round 0.00200, oval 0.00210, square cushion 0.00235; over 5,000 calibrated amethysts and citrines; coloured stones cut with steeper crowns, pavilion bulges and thicker girdles; worked examples; 10 to 15% vs 5% accuracy)
- GIA Gem Encyclopedia: Ruby (corundum, specific gravity 4.00 ±0.05)
- GIA Gem Encyclopedia: Sapphire (corundum, specific gravity 4.00)
- GIA Gem Encyclopedia: Diamond (specific gravity 3.52 ±0.01)
- GIA: Ruby quality factors (commonly cut as ovals and cushions; cutters keep weight; shallow stones show windows)
- GIA, Gems & Gemology, Winter 1997: Synthetic moissanite, a new diamond substitute (a 6.50 mm round brilliant diamond weighs about 1.00 ct; moissanite of the same diameter and cut about 0.91 ct)
- GIA 4Cs: Nine things about diamond carat weight you need to know (one carat is 0.20 g; weight hidden in or below the girdle)
- Handbook of Mineralogy (Mineralogical Society of America): Corundum (measured density 3.98 to 4.10)
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