A liquid inclusion is an internal feature within a gemstone in which a cavity, usually small and irregularly shaped, contains a liquid phase. The liquid is typically water with dissolved salts, although hydrocarbon liquids and liquid carbon dioxide are also encountered in specific geological contexts. Liquid inclusions are part of the broader category of fluid inclusions, which also includes cavities containing gas alone or both gas and liquid phases together. A two-phase inclusion contains both gas and liquid; a three-phase inclusion contains gas, liquid and a solid mineral phase.

Liquid inclusions are diagnostic features in gemmology because they record the chemistry of the geological fluids from which the gem grew. They often distinguish natural from synthetic stones, since most synthetic methods do not produce the same kinds of fluid inclusions as natural geological growth. They can also provide origin clues: emeralds from particular deposits, for instance, characteristically contain three-phase inclusions of distinct geometry and content. Heat treatment alters or destroys some types of liquid inclusion, and the presence or absence of intact unaltered fluid inclusions is one of the indications used to distinguish heated from unheated corundum.

Under the microscope, a liquid inclusion typically appears as a darker outline against the surrounding crystal, sometimes with a visible meniscus separating a liquid from a gas bubble within the cavity, and sometimes with daughter crystals of salts or other dissolved minerals visible along the cavity walls.