Brilliance is the white light that a cut stone returns to the viewer. In our census, it is the share of entering light that leaves through the crown toward the viewer.
In more detail
Brilliance depends on the angles of the cut and on the refractive index of the stone. Our tools count it in different ways, so their figures are not interchangeable. The T73 in diamond reads 93 in our census and 54% in the Cut Grading Tool. Compare census brilliance only between cuts in the same gemstone.
How we measure it
The light census traces 3,200 rays per gemstone. Brilliance is 100 × the useful rays ÷ the rays that enter. A useful ray leaves through the crown within 64° of vertical, after at least one pavilion reflection. The Cut Grading Tool's brilliance is the ISO figure with a 23° head shadow, averaged over 13 tilts.
Related terms
- Fire: Fire is the flashes of spectral color that a cut stone sends out. In our census, fire is the share of returned light whose colors leave more than 0.6° apart.
- Scintillation: Scintillation is the pattern of light and dark flashes a stone shows as it, the light or the viewer moves. Our score reads it from the number of facets that reach the surface of the cut.
- Light census: The light census is our ray tracer behind the Cut Performance Wheel, the cut-page scores and the Cut Performance Index. It drops a fixed set of rays on a stone from above and counts where each one leaves.
- Census ray classes: In our light census, every ray that enters a stone ends in one of five classes. The classes are useful, blink, leak, grazing and lost.
- Cut Grading Tool: The Cut Grading Tool is a free Digital Gemology tool that grades a round brilliant diamond's cut from its proportions. It reads GIA's published cut grade estimation tables; it is for education and does not give a GIA grade.
- Light performance: Light performance is how well a cut stone handles light. We describe it with brilliance (light returned), fire (color) and scintillation (sparkle).