What is Corundum?
{{Infobox mineral | name = Corundum | category = [[Oxide mineral]] – [[Hematite]] group | boxwidth = | boxbgcolor = | image = Several corundum crystals.jpg | caption = | formula = [[Aluminium oxide]], {{chem|[[Aluminium|Al]]|2|[[Oxygen|O]]|3}} | strunz = 04.CB.05 | dana = 4.3.1.1 | symmetry = Trigonal ({{overline|3}}2/m) | unit cell = a = 4.75 Å, c = 12.982 Å; Z=6 | color = Colorless, gray, brown; pink to red, orange, yellow, green, blue, violet; may be color zoned, asteriated mainly grey and brown | habit = Steep bipyramidal, tabular, prismatic, rhombohedral crystals, massive or granular | system = [[Trigonal]] (Hexagonal Scalenohedral) <br/>Symbol ({{overline|3}}2/m) <br/>[[Space group]]: R{{overline|3}}c | twinning = Polysynthetic twinning common | cleavage = None – parting in 3 directions | tenacity = Brittle | fracture = Conchoidal to uneven | mohs = 9 (defining mineral)<ref>{{cite web|title=Mohs' scale of hardness|url=http://www.minsocam.org/msa/collectors_corner/article/mohs.htm|work=Collector's corner|publisher=Mineralogical Society of America|access-date=10 January 2014}}</ref> | luster = Adamantine to vitreous | refractive = n<sub>ω</sub> = 1.767–1.772 <br/>n<sub>ε</sub> = 1.759–1.763 | opticalprop = Uniaxial (–) | pleochroism = None | streak = White | diaphaneity = Transparent, [[translucent]] to [[Opacity (optics)|opaque]] | gravity = 3.95–4.10 | melt = 2044 °C | fusibility = Infusible | diagnostic = | solubility = Insoluble | other = May fluoresce or phosphoresce under UV light | alteration = May alter to mica on surfaces causing a decrease in hardness | references = <ref name=Handbook>{{cite book|editor=Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W. and Nichols, Monte C. |title= Handbook of Mineralogy|publisher= Mineralogical Society of America|place= Chantilly, VA, US|url=http://rruff.geo.arizona.edu/doclib/hom/corundum.pdf|chapter=Corundum|isbn=0962209724 |volume=III(H