Bowtie effect (black shadow in the middle)
The bowtie effect is a black "stripe" through the center half of certain diamond cuts = shadows inside the diamond , created when light casts a shadow on the central facets. The reason for this is that it's hard to avoid when cutting elongated shapes - their center sections are too long, and it can be difficult to grind the facets there to properly reflect the light. This black streak is not due to any particular chemical coloring inside the stone, nor is it due to any particular inclusion. It also has nothing to do with "light leakage" through the sides of the stone, which is one of the more common problems with diamonds.
The diamond bowtie effect is not always pitch black. Many loops are just slight shadows in the center of the stone that are barely noticeable. Almost all marquise, oval and pear shape diamonds have at least some loops, but they are often simply seamless and not as obvious. Some square shapes like the beam cut may also show loops.
Certain loops are even considered positive - if the loop effect is not too pronounced, and if the cutter has shaped the stone well around it, the loop can even add a certain sparkle to the stone and enhance its sparkle.
The presence of a "bowtie" is not listed in the report of a diamond. It's important to note that a diamond bow tie is not necessarily a flaw - many fancy cuts will show some signs of a bow tie due to their complex faceting.
It's all about your personal taste!