Inclusions are usually other minerals or rocks, but may also be water, gas or petroleum. Liquid or vapor inclusions are known as fluid inclusions. In the case of amber it is possible to find insects and plants as inclusions.
The analysis of atmospheric gas bubbles as inclusions in ice cores is an important tool in the study of climate change.
A xenolith is a pre-existing rock which has been picked up by a lava flow. Melt inclusions form when bits of melt become trapped inside crystals as they form in the melt.