Metamictization (sometimes called metamiction) is a natural process resulting in the gradual and ultimately complete destruction of a mineral's crystal structure, leaving the mineral amorphous. Affected material is therefore described as metamict.

Certain minerals occasionally contain interstitial impurities of radioactive compounds and elements, and it is the alpha radiation emitted from these compounds that is responsible for degrading a mineral's crystal structure through internal bombardment. Effects of metamictization are extensive: other than negating any birefringence previously present, the process also lowers a mineral's refractive index, hardness, and specific gravity. The mineral's colour is also affected: metamict specimens are usually green, brown or blackish. Further, metamictization diffuses the bands of a mineral's absorption spectrum. Curiously and inexplicably, the one attribute which metamictization does not alter is dispersion. All metamict materials are themselves radioactive, some dangerously so.