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Crown Angle
The light path reflects back into stone due to high crown angles. Note that the high crown angles cause the angle of incidence to fall outside of the 24.5 ° critical angle.
At 30 ° the angle of dispersion is slighter and can only be viewed in a more parallel direction.
At 34.5 ° crown angles the angle of dispersion is greater and is therefore more easily viewed from many directions.
The original ideal angle for the crown is 34.5 °. Here again, the major gem labs do agree that slight variations from this angle will still produce a well-cut diamond. This angle was designed to provide the final escape route for the reflected internal light so that the dispersion and brilliance could be viewed. It was also important to provide an angle great enough for light to enter and exit from more diverse directions. These diverse directions are multiplied when the star and upper girdle facets are added. But the angles could not be so steep that the light reflection from the pavilion traveling back up to the crown would be reflected back (trapped) into the stone.
At 34 1/2 °, the angle of dispersion is at its greatest. At 30 °one would have to look in a more parallel direction to the table of the diamond to observe its dispersion. At 34 1/2 ° the diamond could be tilted further away from parallel and still portray the dispersion of a well cut diamond.
