Glare is light reflected off the diamonds surface.
As a general rule about 20% of the light that falls on a diamond is likely to be returned to your eyes as reflection. This light will be of the same colour as the source (unless the diamond is colored). Diamond has the highest reflectivity of all gemstones, partly because it has the highest refractive index (the two are related), and partly because diamond takes the best polish of all materials because it is so hard. Mineralogists say diamond has the highest luster – ‘adamantine’ (adamantine is Greek for diamond like!).
The remaining 80% of light enters the diamond and reflects around inside the stone until it all comes out. The goal of diamond polishing is to position facets so that most of this ‘refracted’ light leaves via the table or the crown facets. A diamond acts like a hall of mirrors, taking the light via the ‘windows’ or facets on the top of the diamond, reflecting it from mirror like pavilion facets, back out the crown and table ‘windows’. Light that leaves the back or out the pavilion of the diamond is called leakage. In the best proportioned – best case scenario – about 80% of the 80% exits out the top where you can see it as bright facets, returning light from the illuminated sources, to your eye.
Light can be concentrated in intensity; i.e. what emerges is brighter than if you turned and looked directly at the light source itself. Or the beam of light may have been split by refraction and dispersed into a rainbow colour. But if firey colored light is coming to your eye from a particular facet, and that exact same face is reflecting light (glare) from a brighter more intense source, you may not be able to distinguish the fire because of the glare. So while glare is ever present and good, it not particularly dependant on the proportions of the crown facets, but it must never the less be taken into account by the poor scientists trying to understand these complex issues.