Two sapphires can carry the same certificate, the same carat weight, and the same stated colour, yet look completely different in the hand. The difference is almost always the cut. In coloured stones, cut quality is judged by how a stone handles light — whether it windows, how much it goes dark, and how honestly its weight is distributed — and this guide explains how to assess all three before you buy.
Why Sapphires Are Not Cut Like Diamonds
Diamond cutting is governed by fixed proportion standards, and major labs issue a cut grade from Excellent to Poor. Corundum has no equivalent. A GIA or GRS report will describe a sapphire's cut only descriptively — "modified brilliant cut, oval" — and offer no judgement about whether it was cut well. That evaluation is left entirely to the buyer.
There is a good reason no universal standard exists. The ideal proportions for a sapphire depend on the stone's own colour. A pale Ceylon blue needs a deeper pavilion so light travels a longer path through the crystal and picks up more saturation on the way out. A very dark, iron-rich blue needs the opposite treatment — a shallower pavilion and a larger table — so light exits before the colour turns inky. A single set of proportions applied to both would ruin one of them.
Cutters also have to orient the rough against corundum's optic axis. Sapphire is pleochroic, showing different colours down different crystal directions, so the cutter chooses which face-up colour the finished stone will display. Get the orientation wrong and a fine blue can face up greyish-green regardless of how well the facets are polished.
What a Window Is, and How to Spot One
A window is a transparent patch, usually in the centre of the stone, where light passes straight through the pavilion instead of bouncing back to your eye. It happens when the pavilion is cut too shallow and light strikes the back facets below the critical angle, escaping out the bottom rather than reflecting. The result is a dead zone that shows whatever is behind the stone — your finger, the tray, the setting.
Testing for a window takes five seconds. Hold the stone face-up over a page of printed text in normal light. If you can read words through the middle of it, the stone windows. Tilt it slowly: a well-cut stone stays lively across the tilt, while a windowed one shows a pale, washed-out patch that swims around the centre.
A very small window is tolerable in a pale stone and sometimes even helps it appear brighter. A window covering a significant share of the face-up area is a genuine defect, and trade buyers discount for it accordingly — the stone is carrying weight that contributes nothing to how it looks.
Extinction: The Dark Patches Inside a Stone
Extinction is the opposite failure. These are areas that read as black or grey rather than as colour, caused by a pavilion cut too deep, misaligned facets, or simply a dark, highly saturated body colour that swallows light in the thicker sections.
Some extinction is normal, and a stone with none at all often looks flat. Contrast between bright and dark facets is what the trade calls life. The question is proportion and consistency: if more than roughly a third of the face-up area stays dark under mixed lighting and does not brighten when you tilt the stone, the cut is working against the material.
Windowing and extinction can appear in the same stone — a shallow centre with heavy darkness at the shoulders is a common signature of rough that was cut to preserve weight rather than to perform.
Bulge, Belly, and the Weight Retention Problem
Rough corundum is bought and sold by the carat, and a cutter who loses 15% more weight to achieve better proportions loses real money. The economic pressure runs entirely toward retention, which produces the familiar faults: a bellied pavilion that bulges below the girdle, an overly thick girdle, an off-centre culet, and excessive total depth.
The practical consequence for a buyer is that a 2.00 ct stone can face up like a 1.50 ct stone, with the missing half carat buried beneath the setting where nobody will ever see it. Always check millimetre dimensions against carat weight rather than trusting weight alone. A 2 ct blue sapphire oval should measure somewhere near 8 x 6 mm; if it measures 7 x 5 mm, you are paying for depth.
Symmetry matters in a different way for production buyers. Uneven shoulders and an asymmetric outline are cosmetic in a one-off ring, but they cause real problems in mass-produced settings, which is why calibrated sapphires and calibrated rubies are cut to tighter dimensional tolerances than free-size goods.
How to Judge Cut Before You Buy
- Read-through test: hold the stone over printed text and look for a window.
- Tilt test: rock the stone 20 to 30 degrees and watch whether brightness holds or collapses.
- Millimetres versus carats: compare stated dimensions to the expected size for that weight and shape.
- Ask for video: a short clip under mixed daylight and incandescent light reveals windowing and extinction that a single still photograph hides.
- Check the girdle: a very thick or wavy girdle is a reliable sign of weight-driven cutting.
None of this appears on a certificate, which is exactly why cut is where informed buyers find value. A well-cut stone with a slightly lower colour grade will often outperform a poorly cut stone graded a step higher.
At Thai Gems, we have been cutting sapphire and ruby in Bangkok for more than 70 years, and our cutters are instructed to optimise face-up colour and brilliance rather than maximum weight retention. Browse our current selection of sapphires and ruby solitaires, or contact us for trade pricing and custom cutting to your own specifications.