Editing Zircon (section)

==Gemstone==
[[File:ZirkonBlau.jpg|thumb|right|A pale blue zircon gemstone weighing 3.36 carats]]
[[File:Zircon Bracelet.jpg|thumb|This bracelet has zircon gemstones. The metal is zinc alloy base with silver coating.]]
Transparent zircon is a well-known form of semi-precious [[gemstone]], favored for its high [[specific gravity]] (between 4.2 and 4.86) and adamantine [[Lustre (mineralogy)|luster]]. Because of its high [[refractive index]] (1.92) it has sometimes been used as a substitute for [[diamond]], though it does not display quite the same [[Dispersion (optics)|play of color]] as a diamond. Zircon is one of the heaviest types of gemstone.<ref name="Brauns1912">{{cite book | url=https://books.google.com/books?id=UVaABo4JFgIC&dq=zircon+heaviest&pg=PA217 | title=The Mineral Kingdom (Volume 1) | publisher=J.F. Schreiber | first=Reinhard  | last=Brauns | date=1912 | translator=Leonard James Spencer | page=217}}</ref> Its [[Mohs hardness]] is between that of quartz and topaz, at 7.5 on the 10 point scale, though below that of the similar manmade stone [[cubic zirconia]] (8-8.5). Zircons may sometimes lose their inherent color after long exposure to bright sunlight, which is unusual in a gemstone. It is immune to acid attack except by [[sulfuric acid]] and then only when ground into a fine powder.<ref>{{cite book|title=Gems and Gem Minerals|page=109|author=Oliver Cummings Farrington|date=1903|publisher=A.W. Mumford}}</ref>

Most gem-grade zircons show a high degree of [[birefringence]] which, on stones cut with a table and pavilion cuts (i.e., nearly all cut stones), can be seen as the apparent doubling-up of the latter when viewed through the former, and this characteristic can be used to distinguish them from diamonds and cubic zirconias (CZ) as well as soda-lime glass, none of which show this characteristic. However, some zircons from Sri Lanka display only weak or no birefringence at all, and some other Sri Lanka stones may show clear birefringence in one place and little or none in another part of the same cut stone.<ref>{{cite book|pages=562–563|author=L.J. Spencer|publisher=John Murray|date=1905|title=Report of the Seventy-Fourth Meeting of the British Association for the Advancement of Science}}</ref> Other gemstones also display birefringence, so while the presence of this characteristic may help distinguish a given zircon from a diamond or a CZ, it will not help distinguish it from, for example, a [[topaz]] gemstone. The high specific gravity of zircon, however, can usually separate it from any other gem and is simple to test.

Also, birefringence depends on the cut of the stone in relation to its [[optical axis]]. If a zircon is cut with this axis perpendicular to its table, birefringence may be reduced to undetectable levels unless viewed with a jeweler's [[loupe]] or other magnifying optics. The highest grade zircons are cut to minimize birefringence.<ref name=guide>{{cite web |url=http://www.gemstones-guide.com/Zircon.html#Physical_Optical_Properties_of_Zircon|title=Physical & Optical Properties of Zircon |work=Colored Gemstones Guide|access-date=October 19, 2021}}</ref>

The value of a zircon gem depends largely on its color, clarity, and size. Prior to World War II, blue zircons (the most valuable color) were available from many gemstone suppliers in sizes between 15 and 25 carats; since then, stones even as large as 10 carats have become very scarce, especially in the most desirable color varieties.<ref name=guide/>

Synthetic zircons have been created in laboratories.<ref>{{cite journal|last1=Van Westrenen|first1=Wim|last2=Frank|first2=Mark R. |last3=Hanchar|first3=John M.|last4=Fei|first4=Yingwei|last5=Finch|first5=Robert J.|last6=Zha|first6=Chang-Sheng|title=In situ determination of the compressibility of synthetic pure zircon (ZrSiO4) and the onset of the zircon-reidite phase transition |journal=American Mineralogist|date=January 2004|volume=89|issue=1|pages=197–203|doi=10.2138/am-2004-0123 |bibcode=2004AmMin..89..197V|s2cid=102001496}}</ref> They are occasionally used in jewellery such as earrings. Zircons are sometimes imitated by [[spinel]] and synthetic [[sapphire]], but are not difficult to distinguish from them with simple tools.

Zircon from [[Ratanakiri province]] in Cambodia is heat treated to produce blue zircon gemstones, sometimes referred to by the trade name ''cambolite''.<ref>{{cite web |url=https://www.gemstone.org/blue-zircon-cambolite |title="Blue Zircon Cambolite"}}</ref>