A liquid feather is an internal fracture or fissure in a gemstone whose interior cavity contains a liquid, generally water, mineral solution, or, in some cases, hydrocarbon or carbon dioxide phases. The fissure has the broad, leaf-like outline that gives the inclusion type its trade name of feather; the feather is liquid rather than dry to the extent that its interior is wetted and visibly fluid-bearing under magnification.

Liquid feathers are diagnostic of natural origin in many gemstones, since they record the geological fluids in which the gem grew or to which it was subsequently exposed. They are also indicative of the durability profile of the host stone: a partially healed fissure is structurally weaker than the surrounding crystal, and a stone with extensive liquid feathers is more vulnerable to fracture under thermal or mechanical shock. Heat treatment of corundum, in particular, can alter the appearance of a liquid feather, drying out the cavity, dehydrating the contained mineral phases or producing characteristic discoid disc-shaped fractures around solid inclusions; these features are routinely used by gem laboratories as evidence of heating.

In emerald and other clarity-treated gems, liquid feathers are the surface-reaching fissures into which oils, resins or polymers are introduced as clarity enhancement, and the integrity of the original feather is part of what determines the appearance of the treatment.