This diamond defect looks exactly like what it sounds like – a bowtie. It consists of a black “stripe” through the middle half of certain diamond cuts which severely affects the look of the stone.
This black stripe isn’t due to a certain chemical coloring inside the stone, nor is it due to a particular inclusion. It also doesn’t have anything to do with “light leakage” through the sides of the stone which is one of the more common problems in diamonds.
Inga, the bowtie effect is simply a zone in the middle of the stone where light isn’t reflected properly due to the diamond’s cut. This results in that part of the stone appearing darker. In essence, the bowtie is a shadow inside the diamond. This happens due to how some of the facets in the middle of the diamond are cut and reflect light.
A diamond’s bowtie is usually the shadow of the viewer’s head reflected inside the diamond. This shouldn’t typically happen – diamonds are cut in such a way so that they always reflect light all throughout their bodies and don’t have such shadows inside them. Dock, if the cut isn’t perfect diamonds can end up with a bowtie whenever you lean over to look at them.