Elongated fancy shapes, such as marquise, oval, pear, and radiant cut diamonds often display an effect commonly known as a "bow-tie". A bow-tie is a dark area that runs right to left across the center of the diamond in the approximate shape of a man's bow tie.

The bow-tie itself is caused by light leakage resulting from the misalignment of facets. In this dark area, light entering the diamond is traveling out the pavilion instead of reflecting back to your eye. This creates the dark area you see when looking at the diamond. The diamond itself is not dark, and the bow-tie is not an inclusion.

While most diamonds of these shapes possess some degree of bow tie, it varies in severity from light (near invisible or only visible at certain angles) to severe (large, easily observed from every angle). If the bow tie effect needs to be pointed out to you (or if you are wondering if your diamond has a bow tie) then it is probably quite light. Some degree of bow tie can be a positive as it provides contrast in the diamond; only when it begins to become the focal point (the first thing you notice) should you absolutely rule out the diamond.