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{{Two other uses|crystalline solids|the type of glass|lead crystal}}
[[Image:Unknown Quartz crystal 66.JPG|thumb|[[Quartz]] crystal. The individual grains of this polycrystalline mineral sample are clearly visible.]]
A '''crystal''' or '''crystalline solid''' is a [[solid]] material whose constituent [[atom]]s, [[molecule]]s, or [[ion]]s are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is [[crystallography]]. The process of crystal formation is [[crystallization]].

The word ''crystal'' is derived from the [[ancient Greek]] word κρύσταλλος (''krustallos''), which had the same meaning, but according to the ancient understanding of crystal. At root it means anything congealed by freezing, such as ''[[ice]]''.<ref>{{Citation|year=2000|contribution=kreus-|contribution-url=http://www.bartleby.com/61/roots/IE243.html|title=The American Heritage Dictionary of the English Language: Fourth Edition: Appendix I: Indo-European Roots}}.</ref> The word once referred particularly to [[quartz]], or "rock crystal". 
 
Most metals encountered in everyday life are [[polycrystal]]s. Crystals are often symmetrically intergrown to form [[Crystal twinning|crystal twins]].

==Crystal structure==
[[Image:Insulincrystals.jpg|thumb|[[Insulin]] crystals]]
[[Image:Halite(Salt)USGOV.jpg|thumb|[[Halite]] (sodium chloride) - a single, large crystal]]
The process of forming a crystalline structure from a fluid or from materials dissolved in the fluid is often referred to as '''[[crystallization]]'''. In the ancient example referenced by the root meaning of the word crystal, water being cooled undergoes a phase change from liquid to solid beginning with small ice crystals that grow until they fuse, forming a polycrystalline structure. The physical properties of the ice depend on the size and arrangement of the individual crystals, or grains, and the same may be said of metals solidifying from a molten state. 

Which [[crystal structure]] the [[fluid]] will form depends on the [[chemistry]] of the fluid, the conditions under which it is being solidified, and also on the [[Ambient temperature|ambient pressure]]. While the cooling process usually results in the generation of a crystalline material, under certain conditions, the [[fluid]] may be frozen in a noncrystalline state. In most cases, this involves cooling the fluid so rapidly that [[atoms]] cannot travel to their [[Bravais lattice|lattice]] sites before they lose mobility. A noncrystalline material, which has no [[long-range order]], is called an [[amorphous]], [[vitreous]], or [[glass]]y material. It is also often referred to as an [[amorphous]] solid, although there are distinct differences between solids and glasses: most notably, the process of forming a glass does not release the [[latent heat of fusion]].  For this reason, many scientists consider glassy materials to be [[viscosity|viscous]] liquids rather than [[solid]]s, although this is a controversial topic. 
{{details|glass}} 

Crystalline structures occur in all classes of materials, with all types of [[chemical bond]]s.  Almost all [[metallic bond|metal]] exists in a polycrystalline state; amorphous or single-crystal metals must be produced synthetically, often with great difficulty. [[ionic bond|Ionically bonded]] crystals can form upon solidification of [[salt]]s, either from a [[molten]] fluid or when it condenses from a solution. [[Covalent]]ly bonded crystals are also very common, notable examples being [[diamond]], [[silica]], and [[graphite]]. [[Polymer]] materials generally will form crystalline regions, but the lengths of the molecules usually prevent complete crystallization. Weak [[Van der Waals force]]s can also play a role in a crystal structure; for example, this type of bonding loosely holds together the [[hexagonal]]-patterned sheets in graphite.

Most crystalline materials have a variety of [[crystallographic defect]]s. The types and structures of these defects can have a profound effect on the properties of the materials.

==Crystal phases or forms==
[[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. 

[[Amorphous]] phases are also possible with the same molecule, such as [[amorphous ice]].  In this case, the phenomenon is known as [[polyamorphism]].

For pure chemical elements, polymorphism is known as [[allotropy]].  For example, [[diamond]], [[graphite]], and [[fullerene]]s are different allotropes of [[carbon]].

==Other meanings and characteristics==
[[Image:Monocristal dsc03676.jpg|thumb|A large monocrystal of [[potassium dihydrogen phosphate]] grown from solution by [[Saint-Gobain]] for the [[megajoule laser]] of [[Commissariat à l'Énergie Atomique|CEA]].]]

[[Image:Gallium1 640x480.jpg|thumb|[[Gallium]], a metal that easily forms large single crystals]]
Since the initial discovery of crystal-like individual arrays of atoms that are not regularly repeated, made in 1982 by [[Dan Shechtman]], the  acceptance of the concept and the word [[quasicrystal]] have led the [[International Union of Crystallography]] to redefine the term crystal to mean "any solid having an essentially discrete diffraction diagram", thereby shifting the essential attribute of crystallinity from [[Euclidean space|position space]] to [[Frequency domain|Fourier space]]. Within the family of crystals one distinguishes between traditional crystals, which are periodic, or repeating, at the atomic scale, and aperiodic (incommensurate) crystals which are not. This broader definition adopted in 1996 reflects the current understanding that microscopic periodicity is a sufficient but not a necessary condition for crystals.
[[Image:Ice crystals.jpg|[[Ice]] crystals|thumb|left]]

While the term "crystal" has a precise meaning within [[materials science]] and [[solid-state physics]], colloquially "crystal" refers to solid objects that exhibit well-defined and often pleasing geometric shapes. In this sense of the word, many types of crystals are found in nature. The shape of these crystals is dependent on the types of molecular bonds between the atoms to determine the structure, as well as on the conditions under which they formed. [[Snow]]flakes, [[diamond]]s, and table [[salt]] are common examples of crystals.

Some crystalline materials may exhibit special electrical properties such as the [[ferroelectric effect]] or the [[piezoelectricity|piezoelectric effect]]. Additionally, [[light]] passing through a crystal is often [[Refraction|refracted]] or bent in different directions, producing an [[array]] of [[color]]s; [[crystal optics]] is the study of these effects. In periodic [[dielectric]] structures a range of unique optical properties can be expected as seen in [[photonic crystal]]s.

==Crystalline rocks==
[[Image:A fossil shell with calcite.jpg|thumb|[[Fossil]] [[Animal shell|shell]] with [[calcite]] crystals]]

[[Inorganic]] [[matter]], if free to take that [[physical state]] in which it is most stable, always tends to crystallize. There is no practical limit to the size a crystal may attain under the right conditions, and [[selenite]] single crystals in excess of 10 m are found in the [[Crystal Cave of the Giants]] in Naica, Mexico.<ref> [http://ngm.nationalgeographic.com/2008/11/crystal-giants/shea-text National Geographic, 2008. Cavern of Crystal Giants]</ref> 

Crystalline [[rock (geology)|rock]] masses have [[consolidate]]d from [[aqueous]] [[solution]] or from molten [[magma]]. The vast majority of [[igneous rocks]] belong to this group 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 [[lavas]] were poured out at the surface and cooled very rapidly; in this latter group a small amount of [[amorphous]] or [[glass]]y matter is frequent. Other crystalline rocks, the [[evaporite]]s such as [[rock salt]], [[gypsum]] and some [[limestone]]s have been deposited from aqueous solution, mostly owing to [[evaporation]] in [[arid]] [[climate]]s. Still another group, the [[metamorphic rocks]] which includes the [[marble]]s, [[mica-schist]]s and [[quartzite]]s; are recrystallized, that is to say, they were at first fragmental rocks, like [[limestone]], [[shale]] and [[sandstone]] and have never been in a [[molten]] condition nor entirely in solution. The high temperature and pressure conditions of [[metamorphism]]  have acted on them erasing their original structures, and inducing recrystallization in the solid state.<ref name=EB1911>{{1911|article=Petrology}}</ref>

==Properties==
{| class="wikitable crystals"
|-
! Crystal
! Particles
! Attractive forces
! [[Melting point]]
! Other properties
|-
| [[Ionic crystal|Ionic]]
| Positive and negative [[ion]]s
| [[Electrostatic attraction]]s
| High
| Hard, brittle, good [[electrical conductor]] in molten state 
|-
| [[Molecular crystal|Molecular]]
| [[Chemical polarity|Polar molecules]]
| [[London force]] and [[dipole-dipole attraction]]
| Low
| Soft, [[non-conductor]] or extremely poor conductor of electricity in [[liquid]] state 
|-
| Molecular
| Non-polar molecules
| London force
| Low
| Soft conducter
</div>
|}

==See also==
{{Multicol}}
* [[Amorphous solid]]
* [[Ukichiro Nakaya#Snow Crystal|Artificial Snow Crystal]]
* [[Atomic packing factor]]
* [[Biomineralisation]]
* [[Colloidal crystal]]
* [[Crystal habit]]
* [[Crystal system]]
* [[Crystallite]]
* [[Crystallographic database]]

{{Multicol-break}}
* [[Glass]]
* [[Inorganic Crystal Structure Database (ICSD)|Inorganic Crystal Structure Database]]
* [[Laser Heated Pedestal Growth]]
* [[Lead crystal]]
* [[Liquid crystal]]
* [[Metallic crystal]]
* [[Micro-Pulling-Down]]
* [[Quasicrystal]]
* [[Seed crystal]]
* [[Single crystal]]

{{Multicol-end}}

==References==
{{reflist}}

==External links==
* {{cite web|last=Howard|first=J. Michael|coauthors=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|date=1998|format=html|accessdate=2008-04-20}}
* {{cite web|last=Krassmann|first=Thomas|date=2005–2008|title=The Giant Crystal Project|publisher=Krassmann|format=html|accessdate=2008-04-20|url=http://giantcrystals.strahlen.org}}
* {{cite web|author=Various authors|title=Teaching Pamphlets|publisher=Commission on Crystallographic Teaching|date=2007|url=http://www.iucr.ac.uk/iucr-top/comm/cteach/pamphlets.html|format=html|accessdate=2008-04-20}}
* {{cite web|author=Various authors|title=Crystal Lattice Structures:Index by Space Group|url=http://cst-www.nrl.navy.mil/lattice/spcgrp/|format=html|date=2004|accessdate=2008-04-20|publisher=U.S. Naval Research Laboratory, Center for Computational Materials Science}}

[[Category:Crystals| ]]
[[Category:Fundamental physics concepts]]

[[ar:بلورة]]
[[an:Cristal]]
[[bn:স্ফটিক]]
[[bs:Kristal]]
[[bg:Кристал]]
[[ca:Cristall]]
[[cs:Krystal]]
[[da:Krystal]]
[[de:Kristall]]
[[et:Kristall]]
[[el:Κρύσταλλος]]
[[es:Cristal]]
[[eo:Kristalo]]
[[eu:Kristal]]
[[fa:بلور]]
[[fr:Cristal]]
[[gl:Cristal]]
[[ko:결정]]
[[hi:क्रिस्टल]]
[[hr:Kristal]]
[[io:Kristalo]]
[[bpy:করিসটাল]]
[[id:Kristal]]
[[is:Kristall]]
[[it:Cristallo]]
[[he:גביש]]
[[sw:Fuwele]]
[[la:Crystallum]]
[[lv:Kristāls]]
[[lt:Kristalas]]
[[hu:Kristály]]
[[mk:Кристал]]
[[ml:പരല്‍ (രസതന്ത്രം)]]
[[nl:Kristal (natuurwetenschappen)]]
[[ja:結晶]]
[[no:Krystall]]
[[nn:Krystall]]
[[oc:Cristal]]
[[pl:Ciało krystaliczne]]
[[pt:Cristal]]
[[ro:Cristal]]
[[ru:Кристаллы]]
[[simple:Crystal]]
[[sk:Kryštál]]
[[sl:Kristal]]
[[sr:Кристал]]
[[fi:Kiteinen aine]]
[[sv:Kristall]]
[[ta:படிகம்]]
[[th:ผลึก]]
[[vi:Tinh thể]]
[[tr:Kristal]]
[[uk:Кристали]]
[[yi:קריסטאל]]
[[zh:晶体]]