Cutting ruby (corundum)
Ruby is red corundum. Its refractive index is far lower than diamond's, so its critical angle is much larger, and a pavilion cut for diamond can let light straight out of the back. Corundum designs usually run a steeper pavilion.
Ruby is pleochroic: it shows a purer red along the optic axis and a more orange red across it. Cutters orient the table to the axis where the rough allows, and the shape of the rough often decides between an oval, a cushion or a round.
Ruby optical properties
Where a property has a range, angles are calculated from the lower refractive index, which gives the stricter limit.
Every cut, measured in ruby
For each cut we measured how much of the pavilion, seen face-down, is steeper than ruby's critical angle of 34.6°, so that head-on light reflects instead of leaking out of the back. Weights are for each model at its reference size.
| Cut | Category | Pavilion reflecting head-on light | Weight | Verdict | Files |
|---|---|---|---|---|---|
| BL-Anteros Cut | Brilliants | 100% | 2.17 ct | Holds head-on light | 3D + cutting |
| BL-Vitruvio Cut | Brilliants | 100% | 2.28 ct | Holds head-on light | 3D + cutting |
| Cushion Cut | Jewellery cuts | 100% | 1.72 ct | Holds head-on light | 3D |
| Emerald Cut | Jewellery cuts | 100% | 1.73 ct | Holds head-on light | 3D + cutting |
| Oval Cut | Jewellery cuts | 100% | 1.82 ct | Holds head-on light | 3D |
| Portuguese Bird's Nest Cut | Jewellery cuts | 100% | 2.27 ct | Holds head-on light | 3D + cutting |
| Princess Cut | Jewellery cuts | 100% | 1.65 ct | Holds head-on light | 3D |
| Radiant Square Cut | Jewellery cuts | 100% | 1.73 ct | Holds head-on light | 3D + cutting |
| Square Cut | Jewellery cuts | 100% | 1.77 ct | Holds head-on light | 3D + cutting |
| T105 Round Brilliant Cut | Brilliants | 100% | 2.09 ct | Holds head-on light | 3D + cutting |
| T57 Round Brilliant Cut | Historic cuts | 100% | 2.06 ct | Holds head-on light | 3D + cutting |
| T73 Round Brilliant Cut | Brilliants | 100% | 2.26 ct | Holds head-on light | 3D + cutting |
| Teardrop Cut | Jewellery cuts | 100% | 1.81 ct | Holds head-on light | 3D + cutting |
| Triangle Cut | Jewellery cuts | 100% | 2.27 ct | Holds head-on light | 3D + cutting |
| Trilliant Cut | Jewellery cuts | 100% | 1.75 ct | Holds head-on light | 3D + cutting |
| Old European Cut | Historic cuts | 100% | 2.27 ct | Holds head-on light | 3D + cutting |
| Old Mine Cut | Historic cuts | 100% | 2.27 ct | Holds head-on light | 3D + cutting |
| Mazarin Cut | Historic cuts | 99% | 2.27 ct | Holds head-on light | 3D + cutting |
| Old Single Cut | Historic cuts | 99% | 2.27 ct | Holds head-on light | 3D + cutting |
| Peruzzi Cut | Historic cuts | 99% | 2.27 ct | Holds head-on light | 3D + cutting |
| Heart Cut | Jewellery cuts | 97% | 1.73 ct | Minor windowing | 3D |
| Hexagon Cut | Jewellery cuts | 89% | 1.73 ct | Steepen the pavilion | 3D + cutting |
| Asscher Cut | Jewellery cuts | 89% | 1.73 ct | Steepen the pavilion | 3D + cutting |
| Trillion Cut | Jewellery cuts | 89% | 2.27 ct | Steepen the pavilion | 3D + cutting |
| Octagon Cut | Jewellery cuts | 89% | 2.27 ct | Steepen the pavilion | 3D + cutting |
| Emerald Square Cut | Jewellery cuts | 89% | 1.73 ct | Steepen the pavilion | 3D + cutting |
| Pear Cut | Jewellery cuts | 84% | 1.81 ct | Steepen the pavilion | 3D + cutting |
| Baguette Cut | Jewellery cuts | 78% | 1.73 ct | Steepen the pavilion | 3D + cutting |
| Rectangle Cut | Jewellery cuts | 62% | 1.78 ct | Steepen the pavilion | 3D + cutting |
| Marquise Cut | Jewellery cuts | 54% | 1.80 ct | Steepen the pavilion | 3D + cutting |
| Radiant Cut | Jewellery cuts | 38% | 2.27 ct | Steepen the pavilion | 3D |
Ruby: questions
What pavilion angle should I use for ruby?
For a round design, about 41.5° or steeper. That is the angle at which head-on light meets both sides of the pavilion above ruby's critical angle of 34.6° and reflects twice before leaving through the crown. Fancy shapes vary, which is why every cut page lists its own figure.
What is the critical angle of ruby?
34.6°, from a refractive index of 1.762. Light striking a facet from inside the stone at less than this angle from the facet's normal passes out; above it, it reflects.
Which cuts work best in ruby?
20 of our 31 cuts reflect head-on light from their whole pavilion in ruby, including BL-Anteros Cut, BL-Vitruvio Cut, Cushion Cut, Emerald Cut, Oval Cut, Portuguese Bird's Nest Cut.
How much does ruby weigh compared with diamond?
Ruby has a specific gravity of about 4.00 against diamond's 3.52, so a stone of the same size weighs about 1.14 times as much as the diamond.