Which clarity characteristic would generally have the greatest impact on a gem's marketability?

A large, unhealed feather in a gemstone often has the greatest impact on its marketability due to both its visibility and potential implications for durability. In gemology, feathers are internal fractures that can compromise the structural integrity of the stone. An unhealed feather is particularly concerning because it indicates that the fracture has not closed or been stabilized, making the stone more susceptible to chipping or breaking under stress.

Such a clarity characteristic can significantly detract from the overall appeal of the gem, as buyers typically prioritize durability along with aesthetic qualities. The presence of a large, noticeable feather can lead to a perception of lowered value, as it raises doubts about the stone's longevity and performance in everyday wear.

In contrast, while other inclusions or flaws, such as small healed inclusions, minor surface scratches, or traces of silk, may also influence the stone's appearance and value, they generally do not convey the same level of risk or concern about the gem's structural integrity. Therefore, the presence of a large, unhealed feather stands out as the clarity issue that most adversely impacts marketability.