Any Fracture on or below the surface of a gemstone. Cleavage is a break that occurs in a cleavage plane- a week crystal direction along one or more of a diamond’s four octahedral planes. A fracture is the name for a break in any other direction. Breaks along cleavage planes often have a ribbed or feather-like appearance. A feather might look shiny and white, glossy, or transparent, depending on the viewing direction.
Large feathers, especially larger ones that break the surface of a finished diamond, can present more of a durability problem than any other inclusion because a hard knock can enlarge the fracture. Small feathers are not harmful. They are fully considered in the Clarity Grade of a diamond, and drawn into the Plot on the lab report.