At the end of the 17th century an Italian jeweller, Vincenti Peruzziot, invented the ‘brilliant cut’, which is still used for most diamonds. A brilliant cut diamond is round, with 33 separate facets on its upper surface and 25 on the lower, hidden face. The angles are precisely calculated so that light entering the diamond is reflected internally and emerges again from the top. Poor cutting can spoil this effect.
When Queen Victoria was presented with the 186.5 carat Koh-i-Nor diamond in 1850 she was disappointed by its lack of brilliance and had it re-cut to a final weight of 108.93 carats. The result was better, though still not perfect.
The high refractive index of diamond – the extent to which it ‘bends’ light entering it – means that with proper cutting all the light can be reflected.
A diamond’s brilliance is also caused by the light that enters it being broken up into the separate colours of the spectrum, making its reflections multi-coloured. Because of its extreme hardness, the facets which give diamond its sparkle never wear off.
Picture Credit : Google