The 4Cs: Cut
Cut, not to be confused with shape, is what gives a stone its sparkle. Cut refers to the side-profile of a stone (not the top-view). When a stone is properly cut, or ideal (figure 01 above), it will reflect the maximum amount of light, making the stone look brilliant all around. If its cut is fair or good, it could be either a shallow(02) or deep cut(03). Shallow cut stones allow too much light to shine through the bottom of the stone, which makes for more of grey or muddled sparkle. If the stone is too deep, too much of the light will bounce out the side of the stone, giving the center of the stone a darker appearance.