Cut registry › Jewellery cuts › Triangle
Jewellery cuts · Step cut
Three straight sides.
The triangle is the trillion’s plainer relative: straight sides, few facets, and an outline chosen for how it sits next to something else.
The short version
A triangle cut diamond is a three-sided stone with straight sides and a small number of large facets, distinct from the trillion, which has curved sides and brilliant faceting.
It carries 19 facets on our measurement, the fewest of any shape we publish.
Definition
Triangle and trillion are often used interchangeably and should not be. A trillion is a triangular brilliant, usually with gently convex sides and 31 or 50 facets. A triangle cut in the sense used here has straight sides and step-style faceting, and far fewer facets.
Its role is almost entirely as a side stone. A pair of triangles flanking a centre stone gives a harder, more architectural line than a pair of trillions does, and the straight edges sit flush against a square or emerald-cut centre in a way curved sides cannot.
The three corners are the vulnerability, as on any pointed shape. They are normally slightly bruted rather than left as knife edges, and they still want prongs that cover them.
Proportions
Trade description on the left, our measured reference stone on the right. Triangular cuts are shallow by design, which is why they spread so well for their weight.
| Measure | Commonly cited | Our reference stone | What it does |
|---|---|---|---|
| Outline | equilateral, straight sides | equilateral | Straight, unlike a trillion’s convex sides. |
| Length-to-width | 1.00 to 1.15 | 1.16 | Close to equilateral. |
| Depth | shallow, commonly 35% to 55% | 55.4% | Round-cut depth advice does not transfer. |
| Table | not standardised | 60.0% | Large relative to the stone. |
| Facets | few, often under 25 | 19 | The fewest of any shape we publish. |
No laboratory issues a cut grade for a triangular cut, and at the sizes it is normally used it is rarely certified individually. Triangles are usually bought as matched pairs, specified by millimetre dimensions and a quality range.
Light
Measured on the census that runs on every cut on this site: 3,200 rays face-up, 2,200 at 20° tilt, on exact facet geometry, against T57 as the standard.
| Material | Useful light | Leak | Fire | Scintillation | Global | vs the standard |
|---|---|---|---|---|---|---|
| Diamond | 52.81% | 25.42% | 86.38 | 34.44 | 71.23 | −17.37 |
| Moissanite | 52.22% | 19.61% | 95.66 | 34.44 | 73.21 | −16.61 |
| Cubic zirconia | 53.69% | 27.09% | 94.50 | 34.44 | 72.65 | −19.12 |
| T57 standard, diamond | 92.28% | 1.88% | 78.85 | 71.82 | 88.61 | 0.00 |
The triangle returns 52.81% of entering light usefully and leaks 25.42%, for 71.23 Global against the standard’s 88.61.
Its defining figure is facet count: 19, against the round brilliant’s 57. That is the fewest of any shape on this site, fewer even than the baguette, and it produces 34.44 on scintillation against 71.82. A triangle does not sparkle. It flashes, in a small number of large events.
The corollary is fire, at 86.38 against the standard’s 78.85. Large facets split light into wide, well-separated spectra, so what the triangle does throw, it throws in colour. It also spreads unusually well, reading 103.52 on the Index spread axis from a shallow outline, and holds 94.39 under tilt against 90.41.
Where the light is: useful return across the face
Five equal bands across the stone. The triangle returns 53.6% at the centre against 56.3% at the edges, against the round-brilliant control’s 99.1% and 98.1%.
Against the standard
Facet count. 19 against 57. Everything else on this list follows from that one number.
Sparkle. 34.44 against 71.82, the lowest scintillation figure of any shape we publish.
Fire. 86.38 against 78.85, and the triangle wins, for the same reason it loses on sparkle: its facets are large.
Face-up size. 103.52 on the spread axis. A shallow triangular outline covers a great deal of finger for its weight.
Durability. Three corners against a round’s none. Bruted tips and covering prongs are the answer.
The baseline for all of this is the round brilliant itself. Its facet plan is set out in the anatomy of a round brilliant, and the measured standard we compare against is T57, the stone Tolkowsky published in 1919.
Materials
In moissanite the triangle returns 52.22% for 73.21 Global with fire at 95.66. Very large facets in a high-dispersion material is the strongest colour combination available, and it is why triangular moissanite side stones read so vividly.
In cubic zirconia it returns 53.69% for 72.65 Global.
Compare all three for this shape in the Index.
Trade-offs
Questions
The questions buyers actually ask, answered in full. Where the answer is ours to prove, the figure is measured on the Index; where it is an established gemological fact, it is stated as one.
A triangle cut diamond is a three-sided stone with straight sides and a small number of large facets. It is distinct from the trillion, which is a triangular brilliant with gently convex sides and far more facets.
Sides and faceting. A trillion has convex, gently curved sides and brilliant faceting, at 31 or 50 facets. A triangle has straight sides and step-style faceting, at 19 on our measurement. The trillion scores 80.86 Global in diamond against the triangle’s 71.23, largely because it carries so many more facets.
Few. Our reference stone measures 19 facets reaching the surface, against a round brilliant’s 57, and that is the lowest count of any shape we publish. It is the number behind its scintillation figure of 34.44.
Not in the way a brilliant does. At 34.44 on scintillation against the standard’s 71.82 it produces a small number of large flashes rather than a dense shower of small ones. What it does deliver is fire, 86.38 against 78.85, because large facets throw wide, well-separated spectra.
The three corners are the weak points, as on any pointed shape. They are normally slightly bruted rather than finished as knife edges, and they still need prongs that cover each corner rather than sitting beside it.
Yes. They are cut shallow, so far less of the weight sits below the table, and our reference stone reads 103.52 on the Index spread axis. That axis is derived from depth, so it is a claim about millimetres per carat rather than about outline area.
Almost always as side stones, usually in matched pairs flanking a centre stone. Their straight edges sit flush against a square, emerald or asscher-cut centre in a way a trillion’s curved sides cannot, which is the main reason a designer picks one over the other.
No. GIA issues cut grades for round brilliants only, and at the sizes triangles are normally used they are rarely certified individually at all. They are bought as matched pairs specified by millimetre dimensions and a quality range.
Next
See the whole set on the jewellery cuts index, or the measured house and historic cuts in the cut registry.
Method
Monte-Carlo on exact facet geometry: 3,200 rays face-up plus 2,200 at 20° tilt, cosine-weighted from a 7° near-vertical cone. Useful light is what exits through the crown within 64° of vertical. The same protocol runs on every cut on this site, so the figures are comparable to each other.
They measure our reference geometry for the shape at the proportions listed above, not an average of stones on the market. A triangle cut to different proportions will read differently. Treat them as a like-for-like comparison between shapes, not as a grade for any individual stone.
Engine estimates, ±2 points, not lab-certified grades. We do not publish the Index symmetry axis on these pages: it scores facet regularity within each pavilion family, which is meaningful for a round brilliant and misleading for a shape whose pavilion is deliberately irregular.
34.44 here against the standard’s 71.82. The term is 100(1−e−n/45) on the count of facets that actually reach the surface, so it is a pure function of facet count and nothing else.