Editing Gold (section)

=== Color ===
{{Main|Colored gold}}
[[File:Gold bullion ap 001.JPG|thumb|Gold bars, also called ingots or bullion]]
[[File:Ag-Au-Cu-colours-english.svg|thumb|left|Different colors of [[Silver|Ag]]–Au–[[Copper|Cu]] alloys]]
Whereas most metals are gray or silvery white, gold is slightly reddish-yellow.<ref name="Lippincott-1880">{{cite book |title=Encyclopædia of Chemistry, Theoretical, Practical, and Analytical, as Applied to the Arts and Manufacturers: Glass-zinc |url=https://books.google.com/books?id=o-FYAAAAYAAJ&pg=PA70 |year=1880 |publisher=J.B. Lippincott & Company |pages=70–}}</ref> This color is determined by the frequency of [[plasma oscillation]]s among the metal's valence electrons, in the ultraviolet range for most metals but in the visible range for gold due to [[relativistic quantum chemistry|relativistic effects]] affecting the [[atomic orbital|orbitals]] around gold atoms.<ref>{{cite web |url=http://math.ucr.edu/home/baez/physics/Relativity/SR/gold_color.html |title=Relativity in Chemistry |publisher=Math.ucr.edu |access-date=5 April 2009}}</ref><ref>{{Cite journal |first1=Hubert |last1=Schmidbaur |first2=Stephanie |last2=Cronje |first3=Bratislav |last3=Djordjevic |first4=Oliver |last4=Schuster |journal=Chemical Physics |volume=311 |pages=151–161 |title=Understanding gold chemistry through relativity |doi=10.1016/j.chemphys.2004.09.023 |date=2005 |issue=1–2 |bibcode=2005CP....311..151S}}</ref> Similar effects impart a golden hue to metallic [[caesium]].

Common colored gold alloys include the distinctive eighteen-karat [[rose gold]] created by the addition of copper. Alloys containing [[palladium]] or [[nickel]] are also important in commercial jewelry as these produce white gold alloys. Fourteen-karat gold-copper alloy is nearly identical in color to certain [[bronze]] alloys, and both may be used to produce police and other [[badge]]s. Fourteen- and eighteen-karat gold alloys with silver alone appear greenish-yellow and are referred to as [[green gold]]. Blue gold can be made by alloying with [[iron]], and purple gold can be made by alloying with [[aluminium]]. Less commonly, addition of [[manganese]], [[indium]], and other elements can produce more unusual colors of gold for various applications.<ref name="WorldGoldCouncil" />

[[Colloidal gold]], used by electron-microscopists, is red if the particles are small; larger particles of colloidal gold are blue.<ref>{{Cite book |url=https://books.google.com/books?id=MzT9eWxtmRgC&pg=PA180 |title=Electron Microscopy in Microbiology |date=1988 |publisher=Academic Press |isbn=978-0-08-086049-7}}</ref>