What is Obsidian?
{{Other uses}} [[File:ObsidianOregon.jpg|thumb|right|180px|Obsidian.]] [[File:Rainbow obsidian.jpg|thumb|right|180px|A piece of rainbow obsidian.]] [[File:Glass Mountain.jpg|thumb|right|220px|Glass Mountain, a large obsidian flow at [[Medicine Lake Volcano]].]] [[File:Different rocks at Panum Crater.jpg|thumb|right|200px|Counterclockwise from top: obsidian, [[pumice]] and [[rhyolite]] (light color)]] '''Obsidian''' is a natural [[volcano|volcanic]] [[glass]]: it is a kind of [[igneous]] [[Rock (geology)|rock]]. It is produced when [[lava]] high in [[silica]] (SiO<sub>2</sub>) cools rapidly, and solidifies quickly.<ref>Obsidian is found in rhyolitic lava flows known as ''obsidian flows''. This lava cools rapidly.</ref> Because there is no crystal structure, obsidian [[blade]] edges can reach almost molecular thinness. They were used as [[arrowhead]]s, and today they may be used as surgical [[scalpel]] blades.<ref>{{cite book| last = Wescott| first = David| title = Primitive technology: a book of Earth skills| url = https://books.google.com/books?id=HlwUo0IccoMC&pg=PA90| year = 1999| publisher = Gibbs Smith| isbn = 978-0-87905-911-8| page = 90 }}</ref> == Origin and properties == Obsidian is [[mineral]]-like, but not [[crystal]]line. It is dark in color like [[basalt]]. It consists mainly of SiO<sub>2</sub> ([[silicon dioxide]]), usually 70% or more. Obsidian does not last forever. It is [[phases of matter#Metastable phases|metastable]]: it changes very slowly. Over time, the glass becomes fine-grained crystals. So, no unaltered obsidian is older than [[Cretaceous]] age.<ref>{{cite journal |last1=Marshall |first1=Royal R. |title=Devitrification of Natural Glass |journal=GSA Bulletin |date=1 October 1961 |volume=72 |issue=10 |pages=1493–1520 |doi=10.1130/0016-7606(1961)72[1493:DONG]2.0.CO;2|bibcode=1961GSAB...72.1493M }}</ref> It breaks down faster when water is present. Obsidian has little water,<ref>{{cite web |url=http://www.em.gov.bc.ca/mining/GeolSurv/MetallicMinerals/MineralDepositProfiles/profiles/r12.htm