Editing Crystal (section)

== 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.