Crystallinity refers to the degree of structural order in a solid. In a crystal, the atoms or molecules are arranged in a regular, periodic manner. In a gas, the relative positions of the atoms or molecules are completely random. Amorphous materials, such as liquids and glasses, represent an intermediate case, having order over short distances (a few atomic or molecular spacings) but not over longer distances. For other uses, see Solid (disambiguation). ... Quartz crystal Synthetic bismuth hopper crystal Insulin crystals Gallium, a metal that easily forms large single crystals A huge monocrystal of potassium dihydrogen phosphate grown from solution by Saint-Gobain for the megajoule laser of CEA. In chemistry and mineralogy, a crystal is a solid in which the constituent atoms... A liquid will assume the shape of its container. ... A pair of modern glasses Glasses, also called eyeglasses or spectacles, are frames, bearing lenses worn in front of the eyes normally for vision correction, eye protection, or for protection from UV rays. ...

Many materials (such as glass-ceramics and some polymers), can be prepared in such a way as to produce a mixture of crystalline and amorphous regions. In such cases, crystallinity is usually specified as a percentage of the volume of the material that is crystalline. Even within materials that are completely crystalline, however, the degree of structural perfection can vary. For instance, most metallic alloys are crystalline, but they usually comprise many independent crystalline regions (grains or crystallites) in various orientations separated by grain boundaries; furthermore, they contain other defects (notably dislocations) that reduce the degree of structural perfection. The most highly perfect crystals are silicon boules produced for semiconductor electronics; these are large single crystals (so they have no grain boundaries), are nearly free of dislocations, and have precisely controlled concentrations of defect atoms. Glass-ceramic is a mixture of glass and ceramic materials (mainly lithium-, silicon-, or aluminium-oxides) yielding a material that is impervious to even extreme temperature shocks. ... A polymer (from Greek: πολυ, polu, many; and μέρος, meros, part) is a substance composed of molecules with large molecular mass composed of repeating structural units, or monomers, connected by covalent chemical bonds. ...

Geologists describe four qualitative levels of crystallinity: Holocrystalline rocks are completely crystalline; hypocrystalline rocks are partially crystalline, with crystals embedded in an amorphous or glassy matrix; hypohyaline are partially glassy; and holohyaline rocks (such as obsidian) are completely glassy. Obsidian from Lake County, Oregon Counterclockwise from top: obsidian, pumice and rhyolite (light color) Obsidian is a rock which is a type of naturally occurring glass, produced by volcanoes (igneous origin) when a felsic lava cools rapidly and freezes without sufficient time for crystal growth (see glass transition temperature). ...

Crystallinity can be measured using x-ray diffraction, but calorimetric techniques are also commonly used.