Editing Opal (section)

{{Short description|Hydrated amorphous form of silica}}
{{For|the company|Opel}}
{{Other uses}}
{{Redirect|Opals}}
{{Use dmy dates|date=July 2016}}
{{Infobox mineral
| name        = Opal
| category    = [[Mineraloid]]
| boxwidth    =
| boxbgcolor  =#5cc4e7
| image       = Opal-53714.jpg
| imagesize   = 
| alt         = A blue-green section of opal encased inside a light brown rock
| caption     = A rich seam of iridescent opal encased in matrix
| formula     = [[Hydrate]]d [[silica]]. SiO<sub>2</sub>·''n''H<sub>2</sub>O
| IMAsymbol   = Opl<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3 |pages=291–320|doi=10.1180/mgm.2021.43 |bibcode=2021MinM...85..291W |s2cid=235729616 |doi-access=free}}</ref>
| molweight   =
| color       = Colorless, white, yellow, red, orange, green, brown, black, blue, pink
| habit       = Irregular veins, in masses, in nodules
| system      = Amorphous<ref name="GRG"/>
| twinning    =
| cleavage    = None<ref name="GRG"/>
| fracture    = [[Conchoidal]] to uneven<ref name="GRG"/>
| mohs        = 5.5–6<ref name="GRG"/>
| luster      = Subvitreous to waxy<ref name="GRG"/>
| polish      = Vitreous to resinous<ref name="GRG"/>
| refractive  = {{val|1.450|0.020|0.080}}<br />Mexican opal may read as low as 1.37, but typically reads 1.42–1.43<ref name="GRG"/>
| opticalprop = Single refractive, often anomalous double refractive due to strain<ref name="GRG"/>
| birefringence = none<ref name="GRG"/>
| dispersion  =
| pleochroism = None<ref name="GRG"/>
| fluorescence= black or white body color: inert to white to moderate light blue, green, or yellow in long and short wave, may also phosphoresce, common opal: inert to strong green or yellowish green in long and short wave, may phosphoresce; fire opal: inert to moderate greenish brown in long and short wave, may phosphoresce<ref name="GRG">[[Gemological Institute of America]], ''GIA Gem Reference Guide'' 1995, {{ISBN|0-87311-019-6}}</ref>
| absorption  = green stones: 660&nbsp;nm, 470&nbsp;nm cutoff<ref name="GRG"/>
| streak      = White
| gravity     = {{val|2.15|0.08|0.90}}<ref name="GRG"/>
| density     = 2.09 g/cm<sup>3</sup>
| melt        =
| fusibility  =
| diagnostic  = darkening upon heating
| solubility  = hot [[saline water|salt water]], [[base (chemistry)|bases]], [[methanol]], [[humic acid]], [[hydrofluoric acid]]
| diaphaneity = opaque, translucent, transparent
| other       =
| references  =<ref>{{cite web |url=http://webmineral.com/data/Opal.shtml |title=Opal |publisher=Webmineral |access-date=8 October 2011 |url-status=live |archive-url=https://web.archive.org/web/20111018204256/http://webmineral.com/data/Opal.shtml |archive-date=18 October 2011 |df=dmy-all}}</ref><ref>{{cite web |url=http://mindat.org/min-3004.html |title=Opal |publisher=Mindat.org |access-date=8 October 2011 |url-status=live |archive-url=https://web.archive.org/web/20111006010057/http://www.mindat.org/min-3004.html |archive-date=6 October 2011 |df=dmy-all}}</ref>
}}
[[File:Opale gisements.jpg|thumb|alt=A map of the world displaying the countries where opal is most commonly found; Australia, Ethiopia, Brazil, Mexico and the United States of America are highlighted.|Main opal producing countries]]

'''Opal''' is a [[hydrate]]d [[amorphous]] form of [[silicon dioxide|silica]] (SiO<sub>2</sub>·''n''H<sub>2</sub>O); its [[water content]] may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to the amorphous (chemical) physical structure, it is classified as a [[mineraloid]], unlike [[crystal]]line forms of silica, which are considered [[mineral]]s. It is deposited at a relatively low temperature and may occur in the [[fissure]]s of almost any kind of [[rock (geology)|rock]], being most commonly found with [[limonite]], [[sandstone]], [[rhyolite]], [[marl]], and [[basalt]].

The name ''opal'' is believed to be derived from the [[Sanskrit]] word {{transliteration|sa|upala}} ({{wikt-lang|sa|उपल}}), which means 'jewel', and later the Greek derivative {{transliteration|grc|opállios}} ({{wikt-lang|grc|ὀπάλλιος}}).<ref name=eckert></ref>

There are two broad classes of opal: precious and common. Precious opal displays play-of-color ([[iridescence]]); common opal does not.<ref>{{cite web |title=Opal Description |url=https://www.gia.edu/opal-description |website=Gemological Institute of America |access-date=4 June 2018}}</ref> Play-of-color is defined as "a pseudo chromatic optical effect resulting in flashes of colored light from certain minerals, as they are turned in white light."<ref>{{cite web |title=Glossary: Play of Color |url=https://www.mindat.org/glossary/play_of_color |website=Mindat |access-date=4 June 2018}}</ref> The internal structure of precious opal causes it to [[diffraction|diffract]] light, resulting in play-of-color. Depending on the conditions in which it formed, opal may be transparent, translucent, or opaque, and the background color may be white, black, or nearly any color of the visual spectrum. Black opal is considered the rarest, while white, gray, and green opals are the most common.