Light performance is a working trade term describing the combined optical behaviour of a faceted gemstone, principally a diamond, with respect to how the cut facets capture, redirect and return light to the eye. The term encompasses three traditional sub-properties, brilliance, fire and scintillation, together with several additional descriptors that have been added to the working vocabulary over the past two decades.

The three classical components

Brilliance is the total intensity of white light returned to the observer through the crown facets of the stone. It depends on the cut's ability to internally reflect light through total internal reflection from the pavilion facets and to redirect that light through the table and crown back toward the eye. Fire is the dispersion of returned light into spectral colours, the rainbow flash visible in well-cut diamonds when the stone is moved or the light source moved. Scintillation is the pattern of bright and dark facet reflections seen as the stone or the light source moves; it depends on the size, distribution and contrast of the dark and light areas across the face-up appearance.

Modern measurement

Modern diamond grading laboratories and several proprietary cut-grading systems measure aspects of light performance through reflective imaging. The American Gem Society (AGS) Light Performance Grading System, introduced in 2005 and refined since, uses ray-tracing and a defined illumination model to assign cut grades on the basis of computed light return rather than solely on proportional measurement. The GIA cut grade for round brilliant diamonds, introduced in 2006, is a proportions-based system informed by extensive light-performance modelling, although it is reported as a cut grade rather than as a separate light-performance grade. ASET imaging (Angular Spectrum Evaluation Tool) and IdealScope imaging, both commercially available, provide reflective-image documentation of light performance that is widely used in retail and online channels to communicate cut quality.

Coloured stones

Light performance applies, in modified form, to coloured stones. The optical behaviour of a coloured gem depends additionally on its body colour, on the depth of the pavilion (which influences how much of the stone is filled with colour as light passes through), and on the cut's success in producing a balanced face-up colour. The aesthetic vocabulary differs accordingly: a coloured-stone grader speaks of crystallisation, transparency, brightness, and uniformity of colour rather than of fire and brilliance in the diamond sense.

Working use of the term

For the working bench and retail trade, light performance is most useful as a sales-explanation framework rather than as a single grading metric. A stone with strong light performance returns light evenly, displays vivid scintillation across the face-up area, and produces visible fire under typical viewing conditions. A stone with poor light performance shows dark zones, especially the dead centre or fish-eye effect that follows from a shallow pavilion, weak edge contrast, and muted scintillation. The visual evidence is often clearer in side-by-side comparison of two cut grades than in any single proportions-based number.