Inclusion in mineralogy refers to any material trapped and enclosed within a mineral during its formation. In gemmology, an inclusion is a foreign body inside a gemstone, or a body extending from the interior to the surface. Inclusions may be solid, liquid or gaseous, or a combination of these, and their study can reveal the temperature, pressure, fluid composition and geological environment of mineral growth.
The relationship between inclusions and their host follows James Hutton's law of inclusions: a fragment enclosed by a host rock is older than the host itself. This principle is one of the basic laws of relative dating, together with superposition, original horizontality and cross-cutting relationships, and is used to reconstruct the order of geological events.
The term is also used in metallurgy for non-metallic particles, oxides, sulfides or slag trapped in a metal or alloy. Metallurgical inclusions are usually regarded as defects that can initiate fatigue cracks or concentrate stress, and they are controlled through melting, refining and casting practice.
Mineralogical inclusions are usually fragments of rock or other minerals, and less commonly water, gas or petroleum. Trapped liquids and gases form fluid inclusions, which are important samples of mineralising fluids. Their homogenisation temperature, salinity and composition allow the temperature and pressure of crystallisation to be reconstructed, and they are widely used in ore-deposit and geothermal research.
In amber, insects, plant fragments, pollen and gas bubbles are common inclusions. Amber inclusions provide exceptionally detailed samples for palaeontology, allowing the external morphology, behaviour and ecological associations of ancient organisms to be studied. Atmospheric bubbles in ice cores are key material for palaeoclimate research, because their carbon dioxide, methane and isotopic compositions record past greenhouse-gas concentrations and climate states.
A xenolith is a fragment of pre-existing rock captured and transported by magma, common in basalt and kimberlite. Xenoliths may come from the mantle, lower crust or wall rock and provide direct samples of deep Earth materials. A melt inclusion forms when a small amount of melt is trapped by a growing crystal, and it can be used to analyse the composition of magma at an early stage of evolution.
Inclusions are among the most important factors in gem valuation. They reduce the clarity and value of many stones such as diamond, but increase the value of others, for example the rutile needles in star sapphire that produce asterism, or the three-phase inclusions in some emeralds that help establish origin.
Many coloured stones are expected to contain inclusions that do not significantly affect value. Gemmology commonly divides coloured stones into three clarity types:
Inclusions are also important for separating natural from synthetic stones and for determining origin. Synthetic stones often lack natural inclusion suites or contain characteristic flux, gas bubbles and curved growth striae, while rubies, sapphires and emeralds from different localities have distinctive inclusion fingerprints, such as short rutile needles in Burmese ruby, silk in Kashmir sapphire and three-phase inclusions in Colombian emerald.
Inclusions are also common in metallurgy and metal processing. During melting, oxide, sulfide or nitride particles may enter the liquid metal or form within it and become trapped when the melt solidifies. Inclusions are usually undesirable because particles can initiate fatigue cracks or act as high-stress regions, reducing toughness, fatigue life and machinability.
Metallurgy controls inclusions through raw-material purity, deoxidation practice, slag composition, pouring temperature and solidification rate. Clean-steel technology, electromagnetic stirring, vacuum degassing and filtration reduce inclusion size and number. For critical components, metallography, scanning electron microscopy and ultrasonic testing are used to assess inclusion type, size and distribution.
Inclusion is easily confused with xenolith, pseudomorph and associated minerals. Fluid or melt inclusions are a subset of inclusions, a xenolith is a foreign rock captured by magma, a pseudomorph is a mineral that has been replaced while retaining its original external form, and associated minerals grow together rather than being enclosed.
In identification, the shape, distribution, composition and relationship of inclusions to the host can provide genetic information. Oriented needle inclusions may indicate the growth direction of the host, ring-shaped inclusion arrays may record growth stages, and inclusions along fractures may record later fluid activity.