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{{Short description|Solid material with highly ordered microscopic structure}}
{{redirect|Crystalline|the Björk song|Crystalline (song)}}
{{redirect|Xtal}}
{{about|crystalline solids|other uses|Crystal (disambiguation)}}
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[[File:Quartz 28.jpg|thumb|upright=1.25|Crystals of [[amethyst]] [[quartz]]]]
[[File:Crystalline polycrystalline amorphous2.svg|thumb|upright=1.25|Microscopically, a [[single crystal]] has atoms in a near-perfect [[Periodic function|periodic]] arrangement; a [[polycrystal]] is composed of many microscopic crystals (called "[[crystallite]]s" or "grains"); and an [[amorphous]] solid (such as [[glass]]) has no periodic arrangement even microscopically.]]

A '''crystal''' or '''crystalline solid''' is a [[solid]] material whose constituents (such as [[atom]]s, [[molecule]]s, or [[ion]]s) are arranged in a highly ordered microscopic structure, forming a [[crystal lattice]] that extends in all directions.<ref>{{cite web |title=Chem1 online textbook—States of matter |url=http://www.chem1.com/acad/webtext/states/states.html#SEC4 |author=Stephen Lower |access-date=2016-09-19}}</ref><ref>{{cite book |last1=Ashcroft |last2=Mermin |name-list-style=and |title=[[Ashcroft and Mermin|Solid State Physics]] |year=1976}}</ref> In addition, macroscopic [[single crystal]]s are usually identifiable by their [[Geometry|geometrical shape]], consisting of flat [[face (geometry)|faces]] with specific, characteristic orientations. The scientific study of crystals and crystal formation is known as [[crystallography]]. The process of crystal formation via mechanisms of [[crystal growth]] is called [[crystallization]] or [[solidification]].

The word ''crystal'' derives from the [[Ancient Greek]] word {{lang|grc|κρύσταλλος}} ({{transliteration|grc|''krustallos''}}), meaning both "[[ice]]" and "[[Quartz#Varieties (according to color)|rock crystal]]",<ref>[https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Dkru%2Fstallos κρύσταλλος], [[Henry George Liddell]], [[Robert Scott (philologist)|Robert Scott]], ''A Greek-English Lexicon'', on Perseus Digital Library</ref> from {{lang|grc|κρύος}} ({{transliteration|grc|''kruos''}}), "icy cold, frost".<ref>[https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Dkru%2Fos κρύος], Henry George Liddell, Robert Scott, ''A Greek-English Lexicon'', on Perseus Digital Library</ref><ref>{{cite web |title=crys·tal |work=The American Heritage Dictionary of the English Language |url=https://www.ahdictionary.com/word/search.html?q=crystal&submit.x=0&submit.y=0 |access-date=2023-06-17}}</ref>

Examples of large crystals include [[snowflake]]s, [[diamond]]s, and [[table salt]]. Most inorganic solids are not crystals but [[polycrystal]]s, i.e. many microscopic crystals fused together into a single solid. Polycrystals include most [[metals]], rocks, [[ceramics]], and [[ice]]. A third category of solids is [[amorphous solid]]s, where the atoms have no periodic structure whatsoever. Examples of amorphous solids include [[glass]], [[wax]], and many [[plastic]]s.

Despite the name, [[lead glass|lead crystal, crystal glass]], and related products are ''not'' crystals, but rather types of glass, i.e. amorphous solids.

Crystals, or crystalline solids, are often used in [[pseudoscientific]] practices such as [[crystal therapy]], and, along with [[gemstone]]s, are sometimes associated with [[Spell (paranormal)|spellwork]] in [[Wicca]]n beliefs and related religious movements.<ref>Regal, Brian. (2009). ''Pseudoscience: A Critical Encyclopedia''. Greenwood. p. 51. {{ISBN|978-0-313-35507-3}}</ref><ref>{{cite web |url=http://paganwiccan.about.com/od/spellworkfolkmagic/ss/Magical-Crystals-And-Gemstones.htm#showall |title=Using Crystals and Gemstones in Magic |date=31 August 2016 |author=Patti Wigington |website=[[About.com]] |access-date=14 November 2016 |archive-date=15 November 2016 |archive-url=https://web.archive.org/web/20161115193403/http://paganwiccan.about.com/od/spellworkfolkmagic/ss/Magical-Crystals-And-Gemstones.htm#showall |url-status=dead }}</ref><ref>{{cite web |url=http://witcheslore.com/bookofshadows/healing/the-magic-of-crystals-and-gemstones/4765/ |title=The Magic of Crystals and Gemstones |date=14 December 2011 |website=WitchesLore |access-date=14 November 2016}}</ref>

== Crystal structure (microscopic) ==
{{multiple image
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| header    = Halite (table salt, NaCl): Microscopic and macroscopic
| image1    = Sodium-chloride-3D-ionic.png
| width1    = 100
| alt1      = Halite crystal (microscopic)
| caption1  = Microscopic structure of a [[halite]] crystal. (Purple is [[sodium]] ion, green is [[chlorine]] ion). There is [[cubic crystal system|cubic symmetry]] in the atoms' arrangement
| image2    =Selpologne.jpg
| width2    = 100
| alt2      = Halite crystal (Macroscopic)
| caption2  = Macroscopic (~16 cm) halite crystal. The right-angles between crystal faces are due to the cubic symmetry of the atoms' arrangement}}
{{Main article|Crystal structure}}
The scientific definition of a "crystal" is based on the microscopic arrangement of atoms inside it, called the [[crystal structure]]. A crystal is a solid where the atoms form a periodic arrangement. ([[Quasicrystal]]s are an exception, see [[#Quasicrystals|below]]).

Not all solids are crystals. For example, when liquid water starts freezing, the phase change begins with small ice crystals that grow until they fuse, forming a ''[[polycrystalline]]'' structure. In the final block of ice, each of the small crystals (called "[[crystallite]]s" or "grains") is a true crystal with a periodic arrangement of atoms, but the whole polycrystal does ''not'' have a periodic arrangement of atoms, because the periodic pattern is broken at the [[grain boundaries]]. Most macroscopic [[inorganic]] solids are polycrystalline, including almost all [[metal]]s, [[ceramic]]s, [[ice]], [[rocks]], etc. Solids that are neither crystalline nor polycrystalline, such as [[glass]], are called ''[[amorphous solid]]s'', also called [[glass]]y, vitreous, or noncrystalline. These have no periodic order, even microscopically. There are distinct differences between crystalline solids and amorphous solids: most notably, the process of forming a glass does not release the [[latent heat of fusion]], but forming a crystal does.

A crystal structure (an arrangement of atoms in a crystal) is characterized by its ''unit cell'', a small imaginary box containing one or more atoms in a specific spatial arrangement. The unit cells are [[honeycomb (geometry)|stacked]] in three-dimensional space to form the crystal.

The [[crystal structure|symmetry of a crystal]] is constrained by the requirement that the unit cells stack perfectly with no gaps. There are 219 possible crystal symmetries (230 is commonly cited, but this treats chiral equivalents as separate entities), called [[Space group|crystallographic space groups]].<ref>{{Citation|last=|first=|editor-first1=T. R|editor-last1=Welberry|title=International Tables for Crystallography|url=https://doi.org/10.1107/97809553602060000001|work=|year=2021|volume=A|pages=|place=Chester, England|publisher=International Union of Crystallography|doi=10.1107/97809553602060000001|isbn=978-1-119-95235-0|s2cid=146060934|access-date=}}</ref> These are grouped into 7 [[crystal system]]s, such as [[cubic crystal system]] (where the crystals may form cubes or rectangular boxes, such as [[halite]] shown at right) or [[hexagonal crystal system]] (where the crystals may form hexagons, such as [[Ice Ih|ordinary water ice]]).

== Crystal faces, shapes and crystallographic forms==
[[File:Crystal facet formation.svg|thumb|upright=1.6|As a [[halite]] crystal is growing, new atoms can very easily attach to the parts of the surface with rough atomic-scale structure and many [[dangling bonds]]. Therefore, these parts of the crystal grow out very quickly (yellow arrows). Eventually, the whole surface consists of smooth, [[surface energy|stable]] faces, where new atoms cannot as easily attach themselves.]]
Crystals are commonly recognized, macroscopically, by their shape, consisting of flat faces with sharp angles. These shape characteristics are not ''necessary'' for a crystal—a crystal is scientifically defined by its microscopic atomic arrangement, not its macroscopic shape—but the characteristic macroscopic shape is often present and easy to see.

[[Euhedral]] crystals are those that have obvious, well-formed flat faces. [[Anhedral (petrology)|Anhedral]] crystals do not, usually because the crystal is one grain in a polycrystalline solid.

The flat faces (also called [[facet]]s) of a [[euhedral]] crystal are oriented in a specific way relative to the underlying [[crystal structure|atomic arrangement of the crystal]]: they are [[plane (mathematics)|planes]] of relatively low [[Miller index]].<ref>''The surface science of metal oxides'', by Victor E. Henrich, P. A. Cox, page 28, [https://books.google.com/books?id=X6x1MmPisKkC&pg=PA2 google books link]</ref> This occurs because some surface orientations are more stable than others (lower [[surface energy]]). As a crystal grows, new atoms attach easily to the rougher and less stable parts of the surface, but less easily to the flat, stable surfaces. Therefore, the flat surfaces tend to grow larger and smoother, until the whole crystal surface consists of these plane surfaces. (See diagram on right.)

One of the oldest techniques in the science of [[crystallography]] consists of measuring the three-dimensional orientations of the faces of a crystal, and using them to infer the underlying [[crystal system|crystal symmetry]].

{{anchor|Crystallographic form}}
A crystal's '''crystallographic forms''' are sets of possible faces of the crystal that are related by one of the symmetries of the crystal. For example, crystals of [[galena]] often take the shape of cubes, and the six faces of the cube belong to a crystallographic form that displays one of the symmetries of the [[isometric crystal system]]. Galena also sometimes crystallizes as octahedrons, and the eight faces of the octahedron belong to another crystallographic form reflecting a different symmetry of the isometric system. A crystallographic form is described by placing the Miller indices of one of its faces within brackets. For example, the octahedral form is written as {111}, and the other faces in the form are implied by the symmetry of the crystal.

Forms may be closed, meaning that the form can completely enclose a volume of space, or open, meaning that it cannot. The cubic and octahedral forms are examples of closed forms. All the forms of the isometric system are closed, while all the forms of the monoclinic and triclinic crystal systems are open. A crystal's faces may all belong to the same closed form, or they may be a combination of multiple open or closed forms.<ref name=Sinkankas1964>{{cite book |last1=Sinkankas |first1=John |title=Mineralogy for amateurs. |date=1964 |publisher=Van Nostrand |location=Princeton, N.J. |isbn=0442276249 |pages=134–138}}</ref>

A [[Crystal habit|crystal's habit]] is its visible external shape. This is determined by the [[crystal structure]] (which restricts the possible facet orientations), the specific crystal chemistry and bonding (which may favor some facet types over others), and the conditions under which the crystal formed.

== Occurrence in nature ==
[[File:Ice crystals.jpg|thumb|right|[[Ice]] crystals]]
[[File:CalciteEchinosphaerites.jpg|thumb|[[Fossil]] [[Exoskeleton|shell]] with [[calcite]] crystals]]

=== Rocks ===
By volume and weight, the largest concentrations of crystals in the Earth are part of its solid [[bedrock]]. Crystals found in rocks typically range in size from a fraction of a millimetre to several centimetres across, although exceptionally large crystals are occasionally found.  {{As of|1999}}, the world's largest known naturally occurring crystal is a crystal of [[beryl]] from Malakialina, [[Madagascar]], {{convert|18|m|abbr=on}} long and {{convert|3.5|m|abbr=on}} in diameter, and weighing {{convert|380,000|kg|abbr=on}}.<ref>G. Cressey and I. F. Mercer, (1999) ''Crystals'', London, Natural History Museum, page 58</ref>

Some crystals have formed by [[magmatic]] and [[metamorphic]] processes, giving origin to large masses of crystalline [[rock (geology)|rock]]. The vast majority of [[igneous rocks]] are formed from molten magma and the degree of crystallization depends primarily on the conditions under which they solidified. Such rocks as [[granite]], which have cooled very slowly and under great pressures, have completely crystallized; but many kinds of [[lava]] were poured out at the surface and cooled very rapidly, and in this latter group a small amount of amorphous or [[glass]]y matter is common. Other crystalline rocks, the metamorphic rocks such as [[marble]]s, [[mica-schist]]s and [[quartzite]]s, are recrystallized. This means that they were at first fragmental rocks like [[limestone]], [[shale]] and [[sandstone]] and have never been in a [[molten]] condition nor entirely in solution, but the high temperature and pressure conditions of [[metamorphism]] have acted on them by erasing their original structures and inducing recrystallization in the solid state.<ref name=EB1911>{{EB1911 |wstitle=Petrology |volume=21 |first=John Smith |last=Flett|inline=1}}</ref>

Other rock crystals have formed out of precipitation from fluids, commonly water, to form [[druse (geology)|druses]] or [[quartz]] veins. [[Evaporite]]s such as [[halite]], [[gypsum]] and some limestones have been deposited from aqueous solution, mostly owing to [[evaporation]] in arid climates.

=== Ice ===
Water-based [[ice]] in the form of [[snow]], [[sea ice]], and [[glacier]]s are common crystalline/polycrystalline structures on Earth and other planets.<ref>Yoshinori Furukawa, "Ice"; Matti Leppäranta, "Sea Ice"; D.P. Dobhal, "Glacier"; and other articles in Vijay P. Singh, Pratap Singh, and Umesh K. Haritashya, eds., ''Encyclopedia of Snow, Ice and Glaciers'' (Dordrecht, NE: Springer Science & Business Media, 2011). {{ISBN|904812641X}}, 9789048126415</ref> A single [[snowflake]] is a single crystal or a collection of crystals,<ref>{{Cite book|url=https://books.google.com/books?id=jY9ADAAAQBAJ&q=snowflake+is+usually+single+crystal&pg=PA12|title=The Snowflake: Winter's Frozen Artistry|last1=Libbrecht|first1=Kenneth|last2=Wing|first2=Rachel|date=2015-09-01|publisher=Voyageur Press|isbn=9781627887335|language=en}}</ref> while an [[ice cube]] is a [[polycrystal]].<ref>{{Cite book|url=https://books.google.com/books?id=Drs6DwAAQBAJ&q=ice+cube+polycrystal+snow+is+single+crystal&pg=PA78-IA186|title=Snow Engineering 2000: Recent Advances and Developments|last=Hjorth-Hansen|first=E.|date=2017-10-19|publisher=Routledge|isbn=9781351416238|language=en}}</ref> Ice crystals may form from cooling liquid water below its freezing point, such as ice cubes or a frozen lake. [[Frost]], snowflakes, or small ice crystals suspended in the air ([[ice fog]]) more often grow from a [[supersaturated]] gaseous-solution of water vapor and air, when the temperature of the air drops below its [[dew point]], without passing through a liquid state. Another unusual property of water is that it expands rather than contracts when it crystallizes.<ref>''Nucleation of Water: From Fundamental Science to Atmospheric and Additional Applications'' by Ari Laaksonen, Jussi Malila -- Elsevier 2022 Page 239--240</ref>

=== Organigenic crystals ===
Many living [[organisms]] are able to produce crystals grown from an [[aqueous solution]], for example [[calcite]] and [[aragonite]] in the case of most [[mollusc]]s or [[hydroxylapatite]] in the case of [[bone]]s and [[teeth]] in [[vertebrate]]s.

== Polymorphism and allotropy ==
{{Main article|Polymorphism (materials science)|Allotropy}}
The same group of atoms can often solidify in many different ways. [[Polymorphism (materials science)|Polymorphism]] is the ability of a solid to exist in more than one crystal form. For example, water [[ice]] is ordinarily found in the hexagonal form [[Ice Ih|Ice I<sub>h</sub>]], but can also exist as the cubic [[ice Ic|Ice I<sub>c</sub>]], the [[rhombohedral]] [[ice II]], and many other forms. The different polymorphs are usually called different ''[[Phase (matter)|phases]]''.

In addition, the same atoms may be able to form noncrystalline [[Phase (matter)|phases]]. For example, water can also form [[amorphous ice]], while SiO<sub>2</sub> can form both [[fused silica]] (an amorphous glass) and [[quartz]] (a crystal). Likewise, if a substance can form crystals, it can also form polycrystals.

For pure chemical elements, polymorphism is known as [[allotropy]]. For example, [[diamond]] and [[graphite]] are two crystalline forms of [[carbon]], while [[amorphous carbon]] is a noncrystalline form. Polymorphs, despite having the same atoms, may have very different properties. For example, diamond is the hardest substance known, while graphite is so soft that it is used as a lubricant. [[Chocolate]] can form six different types of crystals, but only one has the suitable hardness and melting point for candy bars and confections. Polymorphism in [[steel]] is responsible for its ability to be [[heat treating|heat treated]], giving it a wide range of properties.

[[Polyamorphism]] is a similar phenomenon where the same atoms can exist in more than one [[amorphous solid]] form.

== Crystallization ==
{{Main article|Crystallization|Crystal growth}}
[[File:1-cooling-crystallizer-schladen.JPG|thumb|upright|Vertical [[Crystallization|cooling crystallizer]] in a beet sugar factory.]]
Crystallization is the process of forming a crystalline structure from a fluid or from materials dissolved in a fluid. (More rarely, crystals may be [[Deposition (phase transition)|deposited]] directly from gas; see: [[epitaxy]] and [[frost]].)

Crystallization is a complex and extensively-studied field, because depending on the conditions, a single fluid can solidify into many different possible forms. It can form a [[single crystal]], perhaps with various possible [[phase (matter)|phases]], [[stoichiometries]], impurities, [[Crystallographic defect|defects]], and [[crystal habit|habits]]. Or, it can form a [[polycrystal]], with various possibilities for the size, arrangement, orientation, and phase of its grains. The final form of the solid is determined by the conditions under which the fluid is being solidified, such as the chemistry of the fluid, the [[ambient pressure]], the [[temperature]], and the speed with which all these parameters are changing.

Specific industrial techniques to produce large single crystals (called ''[[boule (crystal)|boules]]'') include the [[Czochralski process]] and the [[Bridgman technique]]. Other less exotic methods of crystallization may be used, depending on the physical properties of the substance, including [[hydrothermal synthesis]], [[Sublimation (chemistry)|sublimation]], or simply [[Recrystallization (chemistry)|solvent-based crystallization]].

Large single crystals can be created by geological processes. For example, [[Selenite (mineral)|selenite]] crystals in excess of 10&nbsp;[[meter|m]] are found in the [[Cave of the Crystals]] in Naica, Mexico.<ref>{{cite web|url=http://ngm.nationalgeographic.com/2008/11/crystal-giants/shea-text|title=Cave of Crystal Giants |work=National Geographic Magazine |first1=Neil |last1=Shea |date=November 2008 |url-status=dead |archive-url=https://web.archive.org/web/20171219063433/http://ngm.nationalgeographic.com/2008/11/crystal-giants/shea-text |archive-date=Dec 19, 2017 }}</ref> For more details on geological crystal formation, see [[#Rocks|above]].

Crystals can also be formed by biological processes, see [[#Organigenic crystals|above]]. Conversely, some organisms have special techniques to ''prevent'' crystallization from occurring, such as [[antifreeze protein]]s.

== Defects, impurities, and twinning ==
{{Main article|Crystallographic defect|Impurity|Crystal twinning|Mosaicity}}
[[File:Vector de Burgers.PNG|thumb|left|upright=1.25|Two types of crystallographic defects. <u>Top right:</u> [[edge dislocation]]. <u>Bottom right:</u> [[screw dislocation]].]]
An ''ideal'' crystal has every atom in a perfect, exactly repeating pattern.<ref>{{Cite book|url=https://books.google.com/books?id=9fRt0TTYUTgC&q=%22ideal+crystal%22+has+every+atom+in+a+perfect,+exactly+repeating+pattern|title=Report of the Council|last=Britain)|first=Science Research Council (Great|date=1972|publisher=H.M. Stationery Office|language=en}}</ref> However, in reality, most crystalline materials have a variety of [[crystallographic defect]]s, places where the crystal's pattern is interrupted. The types and structures of these defects may have a profound effect on the properties of the materials.

A few examples of crystallographic defects include [[vacancy defect]]s (an empty space where an atom should fit), [[interstitial defect]]s (an extra atom squeezed in where it does not fit), and [[dislocation]]s (see figure at right). Dislocations are especially important in [[materials science]], because they help determine the [[Strength of materials|mechanical strength of materials]].

Another common type of crystallographic defect is an [[impurity]], meaning that the "wrong" type of atom is present in a crystal. For example, a perfect crystal of [[diamond]] would only contain [[carbon]] atoms, but a real crystal might perhaps contain a few [[boron]] atoms as well. These boron impurities change the [[diamond color|diamond's color]] to slightly blue. Likewise, the only difference between [[ruby]] and [[sapphire]] is the type of impurities present in a [[corundum]] crystal.

[[File:Pyrite 60608.jpg|thumb|Twinned [[pyrite]] crystal group.]]
In [[semiconductor]]s, a special type of impurity, called a [[dopant]], drastically changes the crystal's electrical properties. [[Semiconductor device]]s, such as [[transistor]]s, are made possible largely by putting different semiconductor dopants into different places, in specific patterns.

[[Crystal twinning|Twinning]] is a phenomenon somewhere between a crystallographic defect and a [[grain boundary]]. Like a grain boundary, a twin boundary has different crystal orientations on its two sides. But unlike a grain boundary, the orientations are not random, but related in a specific, mirror-image way.

[[Mosaicity]] is a spread of crystal plane orientations. A [[mosaic crystal]] consists of smaller crystalline units that are somewhat misaligned with respect to each other.

== Chemical bonds ==
In general, solids can be held together by various types of [[chemical bond]]s, such as [[metallic bond]]s, [[ionic bond]]s, [[covalent bond]]s, [[van der Waals bond]]s, and others. None of these are necessarily crystalline or non-crystalline. However, there are some general trends as follows:

[[Metal]]s crystallize rapidly and are almost always polycrystalline, though there are exceptions like [[amorphous metal]] and single-crystal metals. The latter are grown synthetically, for example, fighter-jet turbines are typically made by first growing a single crystal of titanium alloy, increasing its strength and melting point over polycrystalline titanium. A small piece of metal may naturally form into a single crystal, such as Type 2 [[telluric iron]], but larger pieces generally do not unless extremely slow cooling occurs. For example, iron [[meteorite]]s are often composed of single crystal, or many large crystals that may be several meters in size, due to very slow cooling in the vacuum of space. The slow cooling may allow the precipitation of a separate phase within the crystal lattice, which form at specific angles determined by the lattice, called [[Widmanstatten pattern]]s.<ref>''Encyclopedia of the Solar System'' by Tilman Spohn, Doris Breuer, Torrence V. Johnson -- Elsevier 2014 Page 632</ref>

[[Ionic compound]]s typically form when a metal reacts with a non-metal, such as sodium with chlorine. These often form substances called salts, such as sodium chloride (table salt) or potassium nitrate ([[saltpeter]]), with crystals that are often brittle and cleave relatively easily. Ionic materials are usually crystalline or polycrystalline. In practice, large [[salt (chemistry)|salt]] crystals can be created by solidification of a [[molten]] fluid, or by crystallization out of a solution. Some ionic compounds can be very hard, such as oxides like [[aluminium oxide]] found in many gemstones such as [[ruby]] and [[synthetic sapphire]].

[[Covalently bonded]] solids (sometimes called [[covalent network solids]]) are typically formed from one or more non-metals, such as carbon or silicon and oxygen, and are often very hard, rigid, and brittle. These are also very common, notable examples being [[diamond]] and [[quartz]] respectively.<ref>[https://www.angelo.edu/faculty/kboudrea/general/formulas_nomenclature/Formulas_Nomenclature.htm#:~:text=Ionic%20compounds%20are%20(usually)%20formed,nonmetals%20react%20with%20each%20other. Angelo State University: Formulas and Nomenclature of Ionic and Covalent Compounds]</ref>

Weak [[van der Waals force]]s also help hold together certain crystals, such as crystalline [[molecular solid]]s, as well as the interlayer bonding in [[graphite]]. Substances such as [[fat]]s, [[lipid]]s and [[wax]] form molecular bonds because the large molecules do not pack as tightly as atomic bonds. This leads to crystals that are much softer and more easily pulled apart or broken. Common examples include chocolates, candles, or viruses. Water ice and [[dry ice]] are examples of other materials with molecular bonding.<ref>''Science for Conservators, Volume 3: Adhesives and Coatings'' by Museum and Galleries Commission -- Museum and Galleries Commission 2005 Page 57</ref>[[Polymer]] materials generally will form crystalline regions, but the lengths of the molecules usually prevent complete crystallization—and sometimes polymers are completely amorphous.

== Quasicrystals ==
[[File:Ho-Mg-ZnQuasicrystal.jpg|thumb|The material [[Holmium–magnesium–zinc quasicrystal|holmium–magnesium–zinc]] (Ho–Mg–Zn) forms [[quasicrystal]]s, which can take on the macroscopic shape of a [[dodecahedron|pentagonal dodecahedron]]. Only quasicrystals can take this 5-fold symmetry.  The edges are 2&nbsp;mm long.]]
{{Main article|Quasicrystal}}
A [[quasicrystal]] consists of arrays of atoms that are ordered but not strictly periodic. They have many attributes in common with ordinary crystals, such as displaying a discrete pattern in [[x-ray diffraction]], and the ability to form shapes with smooth, flat faces.

Quasicrystals are most famous for their ability to show five-fold symmetry, which is impossible for an ordinary periodic crystal (see [[crystallographic restriction theorem]]).

The [[International Union of Crystallography]] has redefined the term "crystal" to include both ordinary periodic crystals and quasicrystals ("any solid having an essentially discrete [[diffraction]] diagram"<ref>{{cite journal |author=International Union of Crystallography |year=1992 |title=Report of the Executive Committee for 1991 |journal=Acta Crystallogr. A |volume=48 |issue= 6|pages=922–946 |doi=10.1107/S0108767392008328|pmc=1826680 }}</ref>).

Quasicrystals, first discovered in 1982, are quite rare in practice. Only about 100 solids are known to form quasicrystals, compared to about 400,000 periodic crystals known in 2004.<ref>{{cite journal|author=Steurer W.|journal= Z. Kristallogr. |volume=219 |year=2004|pages= 391–446|doi=10.1524/zkri.219.7.391.35643|title=Twenty years of structure research on quasicrystals. Part I. Pentagonal, octagonal, decagonal and dodecagonal quasicrystals|issue=7–2004|bibcode = 2004ZK....219..391S |doi-access=free}}</ref> The 2011 [[Nobel Prize in Chemistry]] was awarded to [[Dan Shechtman]] for the discovery of quasicrystals.<ref>{{cite web|url=https://www.nobelprize.org/nobel_prizes/chemistry/laureates/2011/ |title=The Nobel Prize in Chemistry 2011 |publisher=Nobelprize.org |access-date=2011-12-29}}</ref>

== Special properties from anisotropy ==
{{See also|Crystal optics}}
Crystals can have certain special electrical, optical, and mechanical properties that [[glass]] and [[polycrystal]]s normally cannot. These properties are related to the [[anisotropy]] of the crystal, i.e. the lack of rotational symmetry in its atomic arrangement. One such property is the [[piezoelectric effect]], where a voltage across the crystal can shrink or stretch it. Another is [[birefringence]], where a double image appears when looking through a crystal. Moreover, various properties of a crystal, including [[electrical conductivity]], [[electrical permittivity]], and [[Young's modulus]], may be different in different directions in a crystal. For example, [[graphite]] crystals consist of a stack of sheets, and although each individual sheet is mechanically very strong, the sheets are rather loosely bound to each other. Therefore, the mechanical strength of the material is quite different depending on the direction of stress.

Not all crystals have all of these properties. Conversely, these properties are not quite exclusive to crystals. They can appear in [[glass]]es or [[polycrystal]]s that have been made [[anisotropic]] by [[Work hardening|working]] or [[stress (mechanics)|stress]]—for example, [[photoelasticity|stress-induced birefringence]].

== Crystallography ==
{{Main article|Crystallography}}
''[[Crystallography]]'' is the science of measuring the [[crystal structure]] (in other words, the atomic arrangement) of a crystal. One widely used crystallography technique is [[X-ray diffraction]]. Large numbers of known crystal structures are stored in [[crystallographic database]]s.

== Image gallery ==
<gallery heights="200" widths="200">
File:Insulincrystals.jpg|[[Insulin]] crystals [[space manufacturing|grown in earth orbit]]. The low gravity allows crystals to be grown with minimal defects.
File:Hoar frost macro2.jpg|[[Hoar frost]]: A type of ice crystal (picture taken from a distance of about 5&nbsp;cm).
File:Gallium crystals.jpg|[[Gallium]], a metal that easily forms large crystals.
File:Apatite-Rhodochrosite-Fluorite-169799.jpg|An apatite crystal sits front and center on cherry-red rhodochroite rhombs, purple fluorite cubes, quartz and a dusting of brass-yellow pyrite cubes.
File:Monokristalines Silizium für die Waferherstellung.jpg|[[Boule (crystal)|Boules]] of [[silicon]], like this one, are an important type of industrially-produced [[single crystal]].
File:Bornite-Chalcopyrite-Pyrite-180794.jpg|A specimen consisting of a bornite-coated chalcopyrite crystal nestled in a bed of clear quartz crystals and lustrous pyrite crystals. The bornite-coated crystal is up to 1.5&nbsp;cm across.
File:Calcite-millerite association.jpg|Needle-like [[millerite]] crystals partially encased in [[calcite]] crystal and oxidized on their surfaces to [[zaratite]]; from the [[Devonian]] [[Milwaukee Formation]] of [[Wisconsin]]
File:Crystallized sugar, multiple crystals and a single crystal grown from seed.jpg|Crystallized sugar. Crystals on the right were grown from a sugar cube, while the left from a single seed crystal taken from the right. Red dye was added to the solution when growing the larger crystal, but, insoluble with the solid sugar, all but small traces were forced to precipitate out as it grew. 
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== See also ==
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* [[Atomic packing factor]]
* [[Anticrystal]]
* [[Cocrystal]]
* [[Colloidal crystal]]
* [[Crystal growth]]
* [[Crystal oscillator]]
* [[Liquid crystal]]
* [[Time crystal]]
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== References ==
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== Further reading ==
{{Sister project links}}
* {{cite web|last=Howard|first=J. Michael|author2=Darcy Howard (Illustrator)|url=http://www.rockhounds.com/rockshop/xtal/index.html|title=Introduction to Crystallography and Mineral Crystal Systems|publisher=Bob's Rock Shop|year=1998|access-date=2008-04-20|archive-url=https://web.archive.org/web/20060826015700/http://www.rockhounds.com/rockshop/xtal/index.html|archive-date=2006-08-26|url-status=dead}}
* {{cite web|last=Krassmann|first=Thomas|date=2005–2008|title=The Giant Crystal Project|publisher=Krassmann|access-date=2008-04-20|url=http://giantcrystals.strahlen.org|archive-url=https://web.archive.org/web/20080426185221/http://giantcrystals.strahlen.org/|archive-date=2008-04-26|url-status=dead}}
* {{cite web|title=Teaching Pamphlets|publisher=Commission on Crystallographic Teaching|year=2007|url=http://www.iucr.ac.uk/iucr-top/comm/cteach/pamphlets.html|access-date=2008-04-20|archive-url=https://web.archive.org/web/20080417001743/http://www.iucr.ac.uk/iucr-top/comm/cteach/pamphlets.html|archive-date=2008-04-17|url-status=dead}}
* {{cite web|title=Crystal Lattice Structures:Index by Space Group |url=https://www.atomic-scale-physics.de/lattice/spcgrp/index.html |year=2004 |access-date=2016-12-03}}
* {{cite web|title=Crystallography|url=http://www.xtal.iqfr.csic.es/Cristalografia/index-en.html|year=2010|access-date=2010-01-08|publisher=[[Spanish National Research Council]], Department of Crystallography}}

{{Patterns in nature}}
{{Authority control}}

[[Category:Crystals| ]]