Editing Ruby (section)

==Physical properties==
[[Image:Corundum.png|thumb|Crystal structure of rubies]]
Rubies have a [[hardness]] of 9.0 on the [[Mohs scale of mineral hardness]]. Among the natural gems, only [[moissanite]] and [[diamond]] are harder, with diamond having a Mohs hardness of 10.0 and moissanite falling somewhere in between corundum (ruby) and diamond in hardness. Sapphire, ruby, and pure corundum are α-alumina, the most stable form of Al{{sub|2}}O{{sub|3}}, in which 3 electrons leave each aluminium [[ion]] to join the regular octahedral group of six nearby O{{sup|2−}} ions; in pure corundum this leaves all of the aluminium ions with a very stable configuration of no unpaired electrons or unfilled energy levels, and the crystal is perfectly colorless, and transparent except for flaws.

[[Image: Ruby model.jpg|thumb|Crystal structure of ruby showing the substitution of Al{{sup|3+}} ions (blue) with Cr{{sup|3+}} (red).  The substitution density of Cr{{sup|3+}} ions in this model is approximately 2%, approximating the maximum doping normally encountered.<ref>{{cite web| url = https://www.miramodus.com/images.shtml| title = Images of molecular models from Miramodus| access-date = 28 June 2019| archive-date = 24 November 2018| archive-url = https://web.archive.org/web/20181124162526/https://www.miramodus.com/images.shtml| url-status = live}}</ref>]]

When a chromium atom replaces an occasional aluminium atom, it too loses 3 electrons to become a chromium{{sup|3+}} ion to maintain the charge balance of the Al{{sub|2}}O{{sub|3}} crystal.  However, the Cr{{sup|3+}} ions are larger and have [[Electron configuration#Ionization of the transition metals|electron orbitals]] in different directions than aluminium. The octahedral arrangement of the O{{sup|2−}} ions is distorted, and the energy levels of the different orbitals of those Cr{{sup|3+}} ions are slightly altered because of the directions to the O{{sup|2−}} ions.<ref name=causes>{{cite web | url= http://www.webexhibits.org/causesofcolor/6AA.html | title= Ruby: causes of color | access-date= 28 March 2016 | url-status= live | archive-url= https://web.archive.org/web/20160321193313/http://www.webexhibits.org/causesofcolor/6AA.html | archive-date= 21 March 2016 | df= mdy-all }}</ref> Those energy differences correspond to absorption in the ultraviolet, violet, and yellow-green regions of the spectrum.

[[Image:Ruby transmittance.svg |thumb|Transmittance of ruby in optical and near-IR spectra. Note the two broad violet and yellow-green absorption bands and one narrow absorption band at the wavelength of 694 nm, which is the wavelength of the ruby laser.]]

If one percent of the aluminium ions are replaced by chromium in ruby, the yellow-green absorption results in a red color for the gem.<ref name=causes/>  Additionally, absorption at any of the above wavelengths stimulates [[fluorescence|fluorescent]] emission of 694-nanometer-wavelength red light, which adds to its red color and perceived [[Lustre (mineralogy)|luster]].<ref>{{Cite web|title=Ruby Crystal Fluorescence|url=https://physicsopenlab.org/2020/06/15/ruby-crystal-fluorescence/|access-date=4 May 2021|website=PhysicsOpenLab|language=en-US|archive-date=4 May 2021|archive-url=https://web.archive.org/web/20210504014108/https://physicsopenlab.org/2020/06/15/ruby-crystal-fluorescence/|url-status=live}}</ref> The chromium concentration in artificial rubies can be adjusted during the crystal growth process to be ten to twenty times lower than in natural gemstones. [[Theodore Maiman]] says that "because of the low chromium level in these crystals they display a lighter red color than gemstone ruby and are referred to as '''pink ruby'''."<ref name="maiman18">{{cite book |doi=10.1007/978-3-319-61940-8|title=The Laser Inventor|series=Springer Biographies|year=2018|isbn=978-3-319-61939-2}}</ref>

After absorbing short-wavelength light, there is a short interval of time when the crystal lattice of ruby is in an excited state before fluorescence occurs. If 694-nanometer photons pass through the crystal during that time, they can stimulate more fluorescent photons to be emitted in-phase with them, thus strengthening the intensity of that red light. By arranging mirrors or other means to pass  emitted light repeatedly through the crystal, a [[ruby laser]] in this way produces a very high intensity of [[Coherence (physics)|coherent]] red light.

All natural rubies have imperfections in them, including color impurities and inclusions of [[rutile]] needles known as "silk". Gemologists use these needle inclusions found in natural rubies to distinguish them from synthetics, simulants, or substitutes. Usually, the rough stone is heated before cutting. These days, almost all rubies are treated in some form, with heat treatment being the most common practice. Untreated rubies of high quality command a large premium.

Some rubies show a three-point or six-point [[asterism (gemmology)|asterism]] or "star". These rubies are cut into [[cabochons]] to display the effect properly. Asterisms are best visible with a single-light source and move across the stone as the light moves or the stone is rotated. Such effects occur when light is reflected off the "silk" (the structurally oriented [[rutile]] needle inclusions) in a certain way. This is one example where inclusions increase the value of a gemstone. Furthermore, rubies can show color changes—though this occurs very rarely—as well as [[chatoyancy]] or the "cat's eye" effect.

===Versus pink sapphire===
Generally, gemstone-quality corundum in all shades of red, including pink, are called rubies.<ref name="matlins">{{cite book|last=Matlins|first=Antoinette Leonard|title=Colored Gemstones|year=2010|publisher=Gemstone Press|isbn=978-0-943763-72-9|page=203|url=https://books.google.com/books?id=4UANp6MCTSQC|url-status=live|archive-url=https://web.archive.org/web/20160502140108/https://books.google.com/books?id=4UANp6MCTSQC|archive-date=2 May 2016}}</ref><ref>{{cite book|last=Reed|first=Peter|title=Gemmology|year=1991|publisher=Butterworth-Heinemann|isbn=0-7506-6449-5|page=337|url=https://books.google.com/books?id=t-OQO3Wk-JsC|url-status=live|archive-url=https://web.archive.org/web/20170102012607/https://books.google.com/books?id=t-OQO3Wk-JsC|archive-date=2 January 2017}}</ref>  However, in the United States, a minimum color saturation must be met to be called a ruby; otherwise, the stone will be called a [[sapphire|pink sapphire]].<ref name="matlins" />  Drawing a distinction between rubies and pink sapphires is relatively new, having arisen sometime in the 20th century. Often, the distinction between ruby and pink sapphire is not clear and can be debated. As a result of the difficulty and subjectiveness of such distinctions, trade organizations such as the International Colored Gemstone Association (ICGA) have adopted the broader definition for ruby which encompasses its lighter shades, including pink.