Nail Head — Dark-Centred Optical Defect from Excessive Pavilion Depth
Nail Head — Dark-Centred Optical Defect from Excessive Pavilion Depth
The extinction zone produced when a steep pavilion lets light pass through rather than return to the eye
Nail head is an optical defect in faceted gemstones in which the centre of the stone appears dark or opaque when viewed face-up, resembling the dark, rounded head of a nail. It is one of the cardinal cutting defects in coloured gem faceting and the principal failure mode of pavilions cut too deep relative to the optimal angle for the gem's refractive index.
Why it happens
Light entering the crown of a faceted stone is intended to reflect from the pavilion facets and return to the viewer's eye, producing the brilliance of the stone. Reflection at the pavilion is total internal reflection, which requires the angle of incidence on the pavilion facet to exceed the critical angle determined by the gem's refractive index. If the pavilion is cut at the right angle for the species — typically in the 38 to 42 degree range for ruby, sapphire, and tourmaline — incident light reflects efficiently and returns through the crown.
If the pavilion is cut too deep — too steep an angle — light entering the crown strikes the pavilion at angles below the critical angle and refracts through the stone rather than reflecting back. The viewer sees a dark zone at the centre of the stone where light is being lost rather than returned. The dark zone is the nail head.
When it occurs
Nail head most often results from cutting decisions that prioritise weight retention over optical performance. A cutter working a piece of rough that is borderline for thickness will sometimes cut a deep pavilion to retain weight and yield, accepting the optical penalty. The result is a heavier finished stone but one with reduced brilliance and an obvious dark central zone.
Sapphires, tourmalines, and quartz species are particularly prone to nail head when over-cut for weight. Diamonds are less commonly affected because diamond cutting is dominated by the round brilliant pattern with established optimum proportions, but extreme deep diamonds can show a similar central darkness sometimes called 'fish-eye' or 'nail-head' depending on the precise geometry.
Effect on value
Nail head reduces the visual quality of a stone significantly and is reflected in price. A heavily nail-headed stone trades at a sharp discount to a properly proportioned equivalent of the same weight and species, often well below what the per-carat math alone would suggest because the visual impact is severe. Some nail-headed stones can be recut to better proportions at a cost of 10 to 30 per cent of weight; the recut decision depends on the relative value of the original heavier stone versus the smaller, brighter recut.
Identification
Nail head is identified by face-up examination of the stone under standard lighting. A dark central zone that does not respond to tilting and rotation by lighting up is the diagnostic sign. Confirmation comes from measuring pavilion depth and angle against the species' optimum range. See also extinction, window, light leakage.