This term describes either a foreign body, be it liquid, solid or gaseous, that was trapped inside a gem during the course of its formation, or lacunas, i.e. cavities within the stone’s structure. Solid inclusions can be polyhedral crystals, crystalline needles (often found in garnets and quartzes) and bunches of crystalline fibers (such as amphiboles in emeralds).
Inclusions can reduce the value of a gem if they diminish its luster or make it more fragile. They can however give the stone a special attractiveness when they cause asterism or chatoyancy, when they form delicate patterns in agates, or when they combine to form an emerald’s beautiful jardin.
A precious indication of a stone’s past, inclusions are also a means of distinguishing natural gemstones from synthetic imitations.