Editing Peridot (section)

==Gemology==
[[File:Peridot Doubling.jpg|thumb|alt=Due to its high birefringence, doubling of facet junctions is commonly seen when viewing peridot under magnification.|Due to its high birefringence, doubling of facet junctions is commonly seen when viewing peridot under magnification.]]
[[Orthorhombic]] minerals, like peridot, have [[biaxial]] [[birefringence]] defined by three principal axes: {{mvar|α, β}}, and {{mvar|γ}}. Refractive index readings of faceted gems can range around {{mvar|α}} = 1.651, {{mvar|β}} = 1.668, and {{mvar|γ}} = 1.689, with a biaxial positive birefringence of 0.037–0.038. With decreasing magnesium and increasing iron concentration, the specific gravity, color darkness and refractive indices increase, and the {{nobr|{{mvar|β}} index}} shifts toward the {{mvar|γ}} index. Increasing iron concentration ultimately forms the iron-rich [[Endmember|end-member]] of the olivine [[solid solution]] series [[fayalite]].<ref>{{cite journal |last=Koivula |first=John I. |year=1981 |title=San&nbsp;Carlos peridot |journal=[[Gems & Gemology]] |volume=17 |issue=4 |pages=205–214 |doi=10.5741/gems.17.4.205 |url=https://www.gia.edu/doc/San-Carlos-Peridot.pdf |via=gia.edu |access-date=14 February 2023 |lang=en}}</ref>
 
A study of Chinese peridot gem samples determined the hydro-static [[specific gravity]] to be 3.36&nbsp;. The visible-light spectroscopy of the same Chinese peridot samples showed light bands between 493.0–481.0&nbsp;nm, the strongest absorption at 492.0&nbsp;nm.<ref name=Koivula-1986>{{cite journal |last1=Koivula |first1=John I.|last2=Fryer|first2=C.W. |date=1986-04-01 |df=dmy-all |title=The gemological characteristics of Chinese peridot |journal=[[Gems & Gemology]] |volume=22 |issue=1 |pages=38–40 |doi=10.5741/GEMS.22.1.38 |issn=0016-626X |url=http://www.gia.edu/gems-gemology/spring-1986-china-peridot-koivula }}</ref>

The largest cut peridot olivine is a {{convert|310|carat|g|abbr=off|lk=in|adj=on}} specimen in the gem collection of the [[Smithsonian Museum]] in Washington, D.C.

Inclusions are common in peridot crystals but their presence depends on the location where it was found and the geological conditions that led to its crystallization.
* Primary negative crystals – rounded gas bubbles – form in situ with peridot, and are common in Hawaiian peridots.
* Secondary negative crystals form in peridot fractures.
* "Lily pad" cleavages are often seen in San&nbsp;Carlos peridots, and are a type of secondary negative crystal. They can easily be seen under reflected light as circular discs surrounding a negative crystal.
* Silky and rod-like inclusions are common in Pakistani peridots.<ref>{{cite web |title=Sapat Gali, Naran, Kaghan Valley, Mansehra District, Khyber Pakhtunkhwa Province, Pakistan |website=[[mindat.org]] |url=https://www.mindat.org/loc-2536.html }}</ref>
* The most common mineral inclusion in peridot is the chromium-rich mineral [[chromite]].
* Magnesium-rich minerals also can exist in the form of [[pyrope]] and [[magnesiochromite]]. These two types of mineral inclusions are typically surrounded "lily-pad" cleavages. 
* Biotite flakes appear flat, brown, translucent, and tabular.<ref>{{cite dictionary |chapter=Biotite as inclusions |dictionary=Dictionary of Gems and Gemology |year=2009 |pages=87 |publisher=Springer |isbn=978-3-540-72795-8 |doi=10.1007/978-3-540-72816-0_2288 |chapter-url=https://link.springer.com/referenceworkentry/10.1007/978-3-540-72816-0_2288 }}</ref>