It is defined as the intensity of the light that enters the eye after internal and external reflection from the cut gemstone. It is best explained by the terms life, fire and scintillation.
- Life: Incident white light which is reflected as white light only from the surface and from the total internal reflection, of light from its pavilion facets is known as life.
- Fire: Reflection observed as spectral colours is known as fire.
- Scintillation: It is the multiple and alternating reflections of light from the facets of a polished gemstone, when there is a relative movement between the observer and the light source or the gemstone.
Brilliance depends on a number of factors:
- R.I. (critical angle)
- Proportioning, in terms of number and size of facets and their cutting angles.
A cutter must keep in mind that to obtain maximum brilliancy from a gemstone it must be cut in proportions that will cause the greatest possible amount of light entering the stone, to be totally reflected from the pavilion facets and to pass out through the crown facets.
The light which enters the stone must strike the pavilion facets at:
- angles greater than the critical angle to be totally internally reflected.
- then strike the crown facets at angles less than the critical angle to be refracted out and returned to the observer. This is also known as planned leakage of light.
- Light striking a pavilion surface at an angle less than the critical angle will be refracted out of the stone and is this lost to observer, thereby reducing its brilliance. This is known as unplanned leakage of light. This is observed in diamonds clearly in terms of a lumpy stone (deep pavilion) and fish eye (shallow pavilion) stones.