Editing Gypsum (section)

==Crystal varieties==
{{main|Selenite (mineral)}}
Gypsum occurs in nature as flattened and often [[crystal twinning|twinned]] [[crystal]]s, and transparent, cleavable masses called [[Selenite (mineral)|selenite]]. Selenite contains no significant [[selenium]]; rather, both substances were named for the ancient Greek word for the [[Moon]].

Selenite may also occur in a silky, fibrous form, in which case it is commonly called "satin spar". Finally, it may also be granular or quite compact. In hand-sized samples, it can be anywhere from transparent to opaque. A very fine-grained white or lightly tinted variety of gypsum, called [[alabaster]], is prized for ornamental work of various sorts. In arid areas, gypsum can occur in a flower-like form, typically opaque, with embedded sand grains called [[desert rose (crystal)|desert rose]]. It also forms some of the largest crystals found in nature, up to {{convert|12|m|ft|abbr=on}} long, in the form of selenite.<ref>{{cite journal |first1=Juan Manuel |last1= García-Ruiz |first2= Roberto |last2= Villasuso |first3= Carlos |last3= Ayora |first4= Angels |last4= Canals |first5= Fermín |last5= Otálora |title= Formation of natural gypsum megacrystals in Naica, Mexico |journal= Geology |volume=35 |issue=4 |pages=327–330 |year=2007 |doi= 10.1130/G23393A.1|bibcode = 2007Geo....35..327G |url=https://digital.csic.es/bitstream/10261/3439/1/garciaruiznaica.pdf |archive-url=https://web.archive.org/web/20170816122316/http://digital.csic.es/bitstream/10261/3439/1/garciaruiznaica.pdf |archive-date=2017-08-16 |url-status=live |hdl= 10261/3439 |hdl-access= free }}</ref>