A bow tie is an optical shadow effect seen in some fancy shape diamonds like the marquise, oval, pear and heart shapes. The bow tie looks like a man's black bow tie in the middle of the diamond and is due to a misalignment of the pavilion facets causing some to not catch reflected light and appear dark. Very stark bow ties are considered to negatively impact the overall look of the diamond; however, minimal bow ties can actually make a diamond appear brighter and more fiery by creating contrast between the dark and light areas of the diamond.
See Also: Diamond, Marquise, Facet, Cut, Make