Window

A window is a pale, see-through area at the centre of a stone's face-up view, where light passes straight through the pavilion instead of returning to the eye. Hold the stone over a printed page. If you can read the type through the middle, it has a window. The cause is a pavilion cut too shallow, below the angle at which the facets can bounce light back, and the usual reason is weight: flat rough, or a cutter told to keep every point of it. A windowed stone carries carats that add weight but not colour, since the centre shows you the table beneath rather than the stone itself. Extinction is the opposite failure, a stone so deep and dark that facets go black; between the two lies simply good cutting.

Some species suffer more than others. Tourmaline, aquamarine and garnet cut from tabular rough are frequent offenders; native-cut sapphire from Ceylon and Madagascar too. Emerald, cut deep for colour, less so.

In practice

No major laboratory grades cut on a coloured stone report, so a window will not appear on the document. It appears in the numbers. Compare the depth to the width on the measurements line: a stone that is very shallow for its spread has probably been cut for weight, and a good dealer will point at the figures before you ask. At the desk, look at the stone tilted as well as flat, since a small window that vanishes at a few degrees is a different animal from one that sits there whichever way it turns.

Recutting closes a window at a cost in weight, and whether that trade pays depends on the threshold the stone will cross on the way down. A 2.1 ct windowed sapphire that recuts to 1.8 ct is a real decision. That calculation is one a good dealer will do with you openly, and if they will not, do it yourself.