The term light pipe denotes a specific optical defect observed in fashioned diamonds and certain coloured stones, in which a portion of incident light passes straight through the pavilion and exits the back of the gem rather than being internally reflected and returned through the crown. The result, viewed face-up, is a washed-out, glassy zone, often appearing as a darker or paler window in the centre of the table. To the trained eye it is one of the most reliable indicators that a stone has been cut for weight retention rather than optical performance.

Cause and geometry

Light piping is fundamentally a failure of total internal reflection. When a ray strikes a pavilion facet at an angle shallower than the critical angle for that material, it refracts out of the stone instead of reflecting back. In diamond, with a refractive index of 2.417, the critical angle is approximately 24.4 degrees, granting cutters considerable latitude. In quartz, refractive index 1.55, the critical angle is roughly 40 degrees, which is why poorly proportioned amethyst and citrine often display pronounced windows. The shallower the pavilion, the larger the central area through which light pipes.

Round brilliants with pavilion angles below approximately 40 degrees, and fancy shapes with shallow belly zones, are particularly prone. The phenomenon is most easily observed by tilting the stone slightly off-axis under a diffuse light source: the central window will dim and brighten in a manner inconsistent with the surrounding crown facets.

Trade significance

In the diamond trade the appearance of a light pipe is closely tied to the GIA cut grade. Stones graded Excellent rarely exhibit visible piping, while those falling into Good or Fair frequently do. Among coloured stones, where weight retention from rough often takes precedence over ideal proportions, light piping is endemic in commercial-grade aquamarine, kunzite, and tanzanite cut from elongated crystals. A gem that pipes light is, in trade parlance, said to have a fish-eye or to be windowed, and these terms are used interchangeably in many auction catalogues.

Recutting can sometimes close a window by deepening the pavilion, though only at meaningful weight cost. For a third-generation gem house the calculation is straightforward: a 3-carat aquamarine that pipes light may finish at 2.4 carats with a closed pavilion, but the resulting stone will trade at a multiple of the windowed price per carat.

Distinguishing terms

Light pipe should not be confused with extinction (the dark zones in coloured stones caused by light absorption) nor with the deliberate use of fibre-optic light pipes in instrumentation, which is an unrelated engineering term. In the gemmological literature the optical sense is the only one in active use.