Trilliant Cut Diamond Guide: Facets, Ratio, Colour and Settings

What is a trilliant cut diamond?

A trilliant cut diamond is a triangular brilliant: a stone with three sides of equal length, faceted with the same kind of triangular and kite-shaped facets you see on a round brilliant. That brilliant faceting is what separates it from a step-cut triangle, which has long parallel facets running like a staircase and looks more like a mirror than a spark. Both styles appear in our guide to triangle-shaped diamonds in every style. Most trilliants carry between 31 and 50 facets, depending on the cutter and whether the stone is meant to be a centre stone or a side stone.

Trilliants are cut shallow compared with most shapes. A triangle has a lot of surface area for its weight, so a well-cut trilliant looks large for its carat, and the corners throw bright, sharp flashes of light. Diamond is 10 on the Mohs hardness scale, with a refractive index of about 2.42, which is why brilliant faceting returns so much light. Hardness is not the same as toughness, though. Diamond can chip along its cleavage planes if a thin edge takes a knock, and a trilliant has three thin points. Protecting them shapes most of the advice below.

The trilliant belongs to the geometric diamond cuts such as shield, kite and hexagon, and it is one of the shapes in our list of all 34 diamond cuts compared. If any word on this page is new to you, look it up in the GemTrove jewellery glossary.

Trilliant, trillion or trielle: what is the difference?

You will see three names for much the same shape. Trilliant began as a trade name registered by a New York diamond company and has since become the everyday word for any triangular brilliant. Trillion and trielle are other names used by cutters and retailers. On a grading report, GIA usually describes the shape as a triangular modified brilliant, so do not worry if the name in a listing and on the report differ. What matters more is which of the two main styles you are looking at.

• Curved (convex) trilliant: the three sides bow gently outward. The curve adds weight and fullness, softens the outline a little and gives more sparkle across the face. Often cut with around 43 to 50 facets. This is the style used for a trilliant engagement ring, pendants and stud earrings.
• Straight-sided (uncurved) trilliant: the sides are flat, so the stone can sit snugly against the edge of a centre diamond. Often cut with around 31 facets. This is the classic trilliant side stone.
• Concave and step-cut versions: a few cutters curve the sides inward or use step facets. These are much rarer and usually appear in designer pieces.

The difference between curved vs straight trilliant is not just looks. A curved trilliant has more material near each point, which makes the corners a little sturdier. A straight-sided trilliant is thinner at the corners, which is fine once it is set beside a centre stone with claws or a bezel covering its points, but means it should never be worn unprotected.

What are the ideal proportions for a trilliant cut diamond?

The length-to-width ratio compares the stone's length with its width, taken from the measurements on the report. For a trilliant you want an even, equilateral triangle, so the ratio should sit close to 1.00. A trilliant that is noticeably longer than it is wide looks like an arrowhead and will not match a partner stone in a pair.

Because trilliants are shallow, the main cut fault to watch for is a window: a washed-out, see-through area in the middle where light passes straight through the stone instead of bouncing back to your eye. Hold a printed page under the stone, or watch a 360° video against a patterned background. If you can read through the centre, the stone is cut too shallow. A little contrast as the stone moves is normal; a flat, lifeless centre is not.

GIA does not give trilliants an overall cut grade, as it does for round brilliants. You judge the cut from the proportions, the polish and symmetry grades and, above all, how the stone looks. Our page on how diamond cut grades are worked out explains what a cut grade measures, and the difference between ideal and excellent cut labels shows why retailer terms are not the same as a lab grade.

Trilliant diamond size chart: how big is each carat weight?

Because a trilliant is cut shallow and spread wide, it faces up (looks from above) larger than most shapes of the same weight. A 1 carat round brilliant is about 6.4 mm across; a 1 carat curved trilliant is typically about 7 to 7.5 mm along each side. The figures below are typical for a curved trilliant with a ratio of about 1.00. Straight-sided stones of the same weight measure slightly smaller.

Depth changes these numbers a lot. A deeper trilliant carries more weight underneath and looks smaller face-up; a shallower one spreads wider but risks a window. Always compare the millimetre measurements on the report, not just the carat weight. For a solitaire, most people go to at least 0.50 carat so the triangle reads clearly once the claws are in place. For earrings and side stones, 0.10 to 0.30 carat each is common.

What colour and clarity should a trilliant diamond be?

Like most fancy shapes, a trilliant shows body colour more than a round, and the warmth tends to gather in the three corners, where the stone is thinnest and the facets crowd together. In white gold or platinum, most people are happy from H colour or better; if you are sensitive to a warm tint, look at G or higher. In yellow or rose gold, I or J can look just as white, because the metal warms everything around it.

For clarity, aim for an eye-clean stone, meaning no inclusions (internal features) you can see without magnification at normal viewing distance. Brilliant faceting hides small inclusions well, so VS2 or a carefully chosen SI1 is usually enough. The exception is the points. A feather (a small internal crack) near a corner is easier to see and, if it reaches the surface, weakens the thinnest part of the stone. Check the plotting diagram on the report, which maps where each inclusion sits.

Trilliant side stones need one extra check: they must match the centre stone. Side stones within one colour grade of the centre usually look seamless. Two grades apart, the difference can show in daylight, especially if the side stones are whiter than the centre, which makes the centre look warm.

• Solitaire in white metal: G–H colour, VS2–SI1 eye-clean.
• Solitaire in yellow or rose gold: I–J colour can look white, VS2–SI1 eye-clean.
• Side stones: within one colour grade of the centre, eye-clean, and matched to each other.
• Avoid: feathers or chips at any of the three points, and stones that show a window.

What is the best setting for a trilliant diamond ring?

All three points of a trilliant are exposed, so the setting's first job is to cover them. The usual options for a trilliant diamond ring:

• Three V-claws: a V-claw is a V-shaped claw that wraps the point rather than sitting on the edge. One at each corner holds the stone securely and keeps most of the outline visible.
• Bezel or half-bezel: a thin rim of metal around the edge, either all the way round or only at the points. Very secure and suits an active lifestyle, though it hides a little of the outline.
• V-claw or half-bezel seats for side stones: straight-sided trilliants are often set in a V-shaped seat that covers the outer point, with the inner edge sitting against the centre stone.
• Halo: small diamonds around the triangle frame the shape and shield the edges. A triangular halo keeps the sharp look; a rounded halo softens it.

The same thinking applies to any diamond with sharp corners. Our guide to protecting the corners of princess-cut diamonds covers claw types and wear points that apply equally to trilliants.

Plan the wedding band at the same time. Trilliant side stones widen the head of the ring, and the outer V-claws can sit low enough to rub or leave a gap against a straight band. A slightly curved or contoured band, or a straight band paired with a raised setting, usually sits more comfortably. Try the two rings together before the setting is finalised.

Are trilliants good side stones in a three-stone ring?

Yes. Trilliant side stones are one of the most popular ways to frame a centre diamond, because their flat inner edge sits close to the centre and the outer points lead the eye along the finger. They work well with round, oval, cushion, emerald and Asscher centres. Straight-sided trilliants are the usual choice; curved trilliants can be used beside a larger or rounder centre stone.

As a rough guide, each trilliant's side length should be about two-thirds to three-quarters of the centre stone's width. Beside a 1 carat round of about 6.4 mm, that usually means trilliants of around 4.3 to 5 mm, or roughly 0.20 to 0.30 carat each. Smaller than that and they look like an afterthought; larger and they compete with the centre.

Trilliants are not the only option. Our overview of side stone diamond cuts and how they frame a centre compares them all, from the tapered trapezoid cut and the curved half moon cut to the slim bullet cut and the clean lines of baguette side stones. If you are weighing up a softer look, the trilliant vs pear side stones comparison shows the two side by side.

How does a trilliant compare with other diamond shapes?

The closest relatives of the trilliant are the other angular shapes. The shield cut has a similar pointed look with a flat top edge, and the kite cut diamond stretches the idea into a four-sided point. The lozenge cut, a slim diamond-shaped outline, and the six-sided hexagonal cut suit people who like geometry but want fewer sharp corners.

Compared with a round brilliant, a trilliant of the same weight looks larger face-up, costs less per carat and has a bolder, more modern outline, but it needs a more protective setting and more care when matching stones. To see where it sits among every shape, start with our guide to understanding diamond shapes.

How much does a trilliant cut diamond cost in Australia?

Trilliants are cut from both natural and laboratory-grown diamond, and the two look identical to the eye. A lab-grown trilliant diamond is real diamond, with the same hardness and refractive index, grown in a laboratory rather than mined. Lab-grown stones cost much less and are easier to find in larger sizes and in closely matched pairs. Natural trilliants are rarer, especially above 1 carat. Both should come with an independent grading report from a lab such as GIA or IGI, and the report will state clearly whether the stone is natural or lab-grown.

These are broad ranges only; prices move with the market and with the exact grades. A trilliant usually costs less per carat than a round of the same weight, colour and clarity, because demand is lower and trilliants are often cut from flat, triangular rough with little waste. Matched pairs cost more than two single stones of the same total weight, because the cutter or supplier has to find two stones that agree in size, depth, colour and clarity. Carat, colour, clarity and how evenly the triangle is cut move the price most.

How do you choose a trilliant cut diamond? Step by step

1. Decide the role. A centre stone, earrings or pendant usually calls for a curved trilliant; side stones usually call for straight-sided trilliants.
2. Read the report. Confirm the shape, measurements, colour, clarity, polish and symmetry, and whether the diamond is natural or lab-grown.
3. Work out the ratio. Divide length by width from the measurements and aim for about 0.95–1.05.
4. Check for a window. Watch a 360° video against a patterned background; the centre should stay bright, not see-through.
5. Inspect the points. Use the plotting diagram to rule out feathers or chips at any of the three corners.
6. Match pairs carefully. For side stones, compare measurements, depth and colour, and keep them within one colour grade of the centre.
7. Plan the setting and band together. Make sure every point is covered by a V-claw or bezel, and that your wedding band sits comfortably alongside.