feath·er | ˈfe-t͟hər
An internal flaw (inclusion) that has a feathery appearance
A feather is a small crack in a diamond that appears as a jagged line or split in the stone. They can be of different sizes, and are identifiable with a 10x jewelry loupe, but may not be immediately visible from every angle of a gem. These flaws likely date back to the formation of the diamond and are frequently white or transparent, but others colors are possible depending on the type of diamond. Because feathers are original flaws, they are not typically caused by the setting or cutting of the stone, and if they are small enough should not influence the durability of the diamond. Larger feathers or those closer to the surface, however, may present a danger of cracking or chipping if the stone is hit hard enough.