Depth percentage is calculated by dividing the depth by width. When the depth percentage is lower, the diamond will appear larger when set in a ring. When the diamond is too deep, the size of the diamond will not be fully appreciated when set in a ring and the width will only be visible from the diamond's top or most commonly called the diamond's table.
When the diamond is too shallow, it will not show as much brilliance. Diamond's where angles are shallowly cut, allow light to pass through the bottom of the diamond, rather than being reflected back to the top. When a diamond has this characteristic, the lost light usually results in a diamond that would appear dull or glass-like.