Editing Sapphire (section)

==Cause of color==
[[File:Corundum.png|thumb|Crystal structure of sapphire]]
[[File:Sapphire ring.jpg|thumb|Sapphire ring made {{circa|1940}}]]
[[Rubies]] are [[corundum]] with a dominant red body color. This is generally caused by traces of [[chromium]] (Cr<sup>3+</sup>) substituting for the (Al<sup>3+</sup>) ion in the corundum structure. The color can be modified by both iron and trapped hole color centers.<ref>{{cite web |url=http://www.webexhibits.org/causesofcolor/6AA.html |title=Red Rubies |work=Causes of Color |access-date=14 August 2014 |publisher=[[WebExhibits]] online museum |archive-date=9 May 2020 |archive-url=https://web.archive.org/web/20200509120724/http://www.webexhibits.org/causesofcolor/6AA.html |url-status=live }}</ref>

Unlike localized ("intra-atomic") absorption of light, which causes color for chromium and vanadium impurities, blue color in sapphires comes from intervalence charge transfer, which is the transfer of an electron from one transition-metal ion to another via the [[conduction band|conduction]] or [[valence band]]. The iron can take the form Fe<sup>2+</sup> or Fe<sup>3+</sup>, while titanium generally takes the form Ti<sup>4+</sup>. If Fe<sup>2+</sup> and Ti<sup>4+</sup> ions are substituted for Al<sup>3+</sup>, localized areas of charge imbalance are created. An electron transfer from Fe<sup>2+</sup> and Ti<sup>4+</sup> can cause a change in the [[valence (chemistry)|valence]] state of both. Because of the valence change, there is a specific change in energy for the electron, and [[electromagnetic energy]] is absorbed. The [[wavelength]] of the energy absorbed corresponds to yellow light. When this light is subtracted from incident white light, the complementary color blue results. Sometimes when atomic spacing is different in different directions, there is resulting blue-green [[dichroism]].

Purple sapphires contain trace amounts of [[chromium]] and [[iron]] plus [[titanium]] and come in a variety of shades. Corundum that contains extremely low levels of chromophores is near colorless. Completely colorless corundum generally does not exist in nature. If trace amounts of [[iron]] are present, a very pale yellow to green color may be seen. However, if both titanium and iron impurities are present together, and in the correct [[Valence (chemistry)|valence]] states, the result is a blue color.<ref>{{cite web |url=http://www.webexhibits.org/causesofcolor/8.html |title=Blue Sapphire |work=Causes of Color |access-date=14 August 2014 |publisher=[[WebExhibits]] online museum |archive-date=10 May 2020 |archive-url=https://web.archive.org/web/20200510034749/http://www.webexhibits.org/causesofcolor/8.html |url-status=live }}</ref>

Intervalence charge transfer is a process that produces a strong colored appearance at a low percentage of impurity. While at least 1% chromium must be present in corundum before the deep red ruby color is seen, sapphire blue is apparent with the presence of only 0.01% of titanium and iron.

Colorless sapphires, which are uncommon in nature, were once used as diamond substitutes in jewelry, and are presently used as accent stones.<ref name=GIA/>

The most complete description of the causes of color in corundum extant can be found in Chapter&nbsp;4 of ''Ruby & Sapphire: A Gemologist's Guide'' (chapter authored by John Emmett, Emily Dubinsky and Richard Hughes).<ref name="Hughes-2017"/>{{rp|107–164}}