Engagement Rings
Round Brilliant Moissanite Engagement Rings
The most bought shape in the world, and the one most often bought on the wrong number.
Written by the Mapple Gems workshop, Surat Β· Updated 17 August 2026
Short answer
A round brilliant moissanite is a silicon carbide stone cut with 57 or 58 facets, Mohs 9.25, with roughly 2.4Γ the dispersion of diamond. It is sold by millimetre diameter and diamond equivalent weight (DEW), not by true carat weight, because moissanite is about 15% less dense than diamond. A stone described as 1ct round moissanite measures about 6.5Β mm and actually weighs closer to 0.88Β ct.
The round brilliant is the shape almost everyone pictures when they picture an engagement ring, and it is the shape where the most money gets spent on the least understood number. If you take one thing from this guide, take the millimetre rule: with moissanite you buy a size, not a weight. Every honest seller in this category quotes both.
Why moissanite is measured in millimetres
Carat is a unit of weight, not size. Diamond has a specific gravity of about 3.52. Moissanite, which is silicon carbide, sits at about 3.21. Put the same volume of each on a scale and the moissanite comes out lighter.
So the trade quotes moissanite in two ways at once: the physical diameter in millimetres, and the DEW figure, which answers the question βwhat carat diamond would look this big?β If a listing gives you only one of those numbers, ask for the other before you order.
| Diameter | Sold as (DEW) | Actual moissanite weight |
|---|---|---|
| 5.0 mm | 0.50 ct | approx. 0.44 ct |
| 6.5 mm | 1.00 ct | approx. 0.88 ct |
| 7.5 mm | 1.50 ct | approx. 1.32 ct |
| 8.0 mm | 2.00 ct | approx. 1.76 ct |
| 9.5 mm | 3.00 ct | approx. 2.64 ct |
Figures are the industry convention for well-cut rounds. Individual stones vary with depth percentage.
What 57 facets actually do
The modern round brilliant carries 57 facets, or 58 when a small culet is polished at the base. Its proportions were worked out to return light straight back up through the table. It is the only shape that receives a formal cut grade from the Gemological Institute of America, because it is the only one whose geometry is standardised enough to grade.
Applied to moissanite, that geometry does something specific. Moissanite has a refractive index of roughly 2.65 to 2.69 against diamondβs 2.42, and a dispersion of 0.104 against diamondβs 0.044. Dispersion is the splitting of white light into spectral colours β the rainbow flash. Moissanite has roughly 2.4 times more of it.
The honest part. That extra dispersion scales with size. A 5Β mm round moissanite reads as a diamond to almost anyone. A 9.5Β mm one throws visible coloured fire that diamond does not. If your goal is a stone nobody questions, a large moissanite is a worse lookalike than a small one. If your goal is maximum sparkle for the money, that is exactly the point.
Which settings suit a round brilliant
A round is the most forgiving shape there is. It has no points to chip, no bow-tie to avoid, and no orientation to get wrong. That means the setting decision is about your hands and your week, not about protecting the stone.
- Four-prong solitaire β the most light in, the most stone visible. Best if you want the stone to read as large.
- Six-prong β marginally more secure, and the extra prongs visually round the outline further. Worth it above 8Β mm.
- Bezel β the metal rim protects the girdle completely. The right answer for nursing, climbing, hands-on trades. It costs you a little brightness at the edge.
- Hidden halo β adds visual size from above without a visible halo ring. It raises the stone, so check the profile height if you wear gloves.
- Cathedral β shoulders sweep up to hold the head. Elegant, but it can block a wedding band from sitting flush.
Metal: the one thing people get wrong with round moissanite
Because moissanite disperses so strongly, a warm metal in direct contact with the stone will visibly warm the light coming out of it. In yellow or rose gold, a large round can pick up a faint champagne cast at the edges.
The workshop fix is simple and costs nothing extra: set the stone in a white head β white gold or rhodium-finished β and run the band in whatever colour you actually want. You get the warm band you asked for and the stone stays neutral. We build rings this way as a matter of course when the centre stone is over 7Β mm.
What to check before you approve the order
- The millimetre diameter, in writing, not just the DEW figure.
- The metal and karat β 925 sterling silver, or solid 10K, 14K or 18K. If a listing says βgoldβ without a karat, that is a question, not a description.
- Whether the head is white while the band is coloured, if you are going over 7Β mm.
- The profile height from finger to table, if you want a band to sit flush later.
- Video of the actual stone under ordinary indoor light. Studio stills flatter every stone equally; video does not.
Keep reading
- What is moissanite? β the stone itself, from the start
- Education β metals, settings, sizing and buying
- Shop moissanite engagement rings
Frequently asked questions
How many millimetres is a 1 carat round moissanite?
About 6.5Β mm across. That is the diameter of a 1Β ct round diamond, which is why the stone is described as 1Β ct DEW. Its true weight is closer to 0.88Β ct because silicon carbide is less dense than diamond.
Is a round brilliant the best shape for moissanite?
It is the safest. The round brilliant has no points to chip, no bow-tie effect, and standardised proportions, so a badly cut one is easier to spot than in any fancy shape. Elongated shapes such as oval and marquise will look larger for the same weight, but they carry more ways to get it wrong.
Will people know a round moissanite is not a diamond?
Below about 7Β mm, almost nobody can tell by eye. Above that, moissaniteβs higher dispersion produces coloured flashes that a diamond does not throw, and a trained eye will notice in direct light. A jeweller with a loupe can also see doubled facet edges, because moissanite is doubly refractive.
Does a round moissanite scratch or cloud over time?
No. Moissanite is Mohs 9.25, above every mineral in ordinary household dust. The clouding people associate with diamond alternatives happens to cubic zirconia at Mohs 8.5, which is softer than the quartz particles in everyday dust and loses its crisp facet edges over a few years. Moissanite does not have that problem; what looks like cloudiness is a film of soap, lotion or skin oil that washes off.
Where does moissanite come from?
The mineral was identified by the French chemist Henri Moissan in 1893, in rock from the Canyon Diablo meteorite crater in Arizona. Natural silicon carbide is far too rare to cut, so every moissanite sold in jewellery is laboratory grown β which is also why it is conflict-free by construction rather than by certification.
Every ring is made to order in our own Surat workshop.
Tell us the millimetre size, the metal and the setting, and we build it. Nothing is pulled off a shelf.
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