3D model and faceting diagram · 57 facets · 8.15 x 8.15 mm
The 57-facet round brilliant with a knife-edge girdle, cut to the proportions Marcel Tolkowsky published in Diamond Design in 1919. Nothing is added to the facet plan: table, 8 stars, 8 bezels, 16 upper halves, 8 pavilion mains and 16 lower halves.
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The 57-facet round brilliant with a knife-edge girdle, cut to the proportions Marcel Tolkowsky published in Diamond Design in 1919. Nothing is added to the facet plan: table, 8 stars, 8 bezels, 16 upper halves, 8 pavilion mains and 16 lower halves.
It is the last step in the evolution of the brilliant before the faceted girdle became standard: the historical baseline of the modern round, kept for teaching, comparison and period work.
Measured from the delivered model, not quoted from a standard.
The same geometry behaves differently in every material. Light that meets a pavilion facet at less than the stone's critical angle passes straight out of the back instead of reflecting, which shows face-up as a window. For each stone we measured how much of this model's pavilion, as seen face-down, is steeper than that angle, and what the model weighs at its reference size. The pavilion mains of this design sit at 40.8°. As a rule of thumb a round design needs about 43.5° in amethyst for head-on light to reflect twice before it leaves the crown, so the mains are worth steepening for ruby, sapphire, emerald, amethyst.
| Stone | Refractive index | Critical angle | Pavilion reflecting head-on light | Weight at this size | Verdict |
|---|---|---|---|---|---|
| Diamond | 2.417 | 24.4° | 100% | 1.81 ct | Holds head-on light |
| Moissanite | 2.648 to 2.691 | 22.2° | 100% | 1.65 ct | Holds head-on light |
| Cubic Zirconia | 2.15 to 2.18 | 27.7° | 100% | 2.98 ct | Holds head-on light |
| Ruby | 1.762 to 1.770 | 34.6° | 100% | 2.06 ct | Holds head-on light |
| Sapphire | 1.762 to 1.770 | 34.6° | 100% | 2.06 ct | Holds head-on light |
| Emerald | 1.577 to 1.583 | 39.4° | 100% | 1.40 ct | Holds head-on light |
| Amethyst | 1.544 to 1.553 | 40.4° | 100% | 1.36 ct | Holds head-on light |
Head-on light, first reflection. A full light-return census is part of the Brilliani Labs engine and will come to this page.
Six formats: STEP (the exact faceted solid for CAD), 3DM for Rhino, STL and OBJ meshes, FBX for Blender, 3ds Max, Maya, Cinema 4D, Unity and Unreal, and GLB for the web and AR. All are in millimetres at real-world scale, with 57 planar facets.
Yes. The cutting files contain a GemCad .asc design (57 facets, 96 index, 8-fold mirror-image symmetry), a tier table with angles, indices and cutting order in CSV, and a printable faceting diagram.
We measured the pavilion of this model against the critical angle of each stone. In Diamond, Moissanite, Cubic Zirconia, Ruby, Sapphire, Emerald, Amethyst, the whole pavilion reflects head-on light. The pavilion mains of this design sit at 40.8°. As a rule of thumb a round design needs about 43.5° in amethyst for head-on light to reflect twice before it leaves the crown, so the mains are worth steepening for ruby, sapphire, emerald, amethyst.
We check it facet by facet against the 3D model it was written from. Every crown and pavilion facet within 0.030° of the 3D model (average 0.013°). The angles are the ones you set on the machine; the 3D model is the same stone you render or set.
The model measures 8.15 x 8.15 x 4.83 mm, which is 2.00 ct in diamond. The same size weighs 1.65 ct in moissanite, 2.98 ct in cubic zirconia, 2.06 ct in ruby, 2.06 ct in sapphire, 1.40 ct in emerald, 1.36 ct in amethyst. Scale it uniformly to any size: weight follows the cube of the size.
Yes. The mesh is watertight and manifold, with outward normals and no duplicate vertices, so it prints and casts without repair. Use the STEP file when you need the exact faceted solid in CAD.
The T57 Round Brilliant Cut as exact geometry: 57 planar facets, real-world scale, ready for CAD, rendering, printing and casting.
The T57 Round Brilliant Cut as a faceting design: 57 facets, 96 index, 8-fold mirror-image symmetry, ready for the machine.
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