Editing Opal (section)

== Precious opal <span class="anchor" id="Precious_opal_anchor"></span> ==
[[File:Opal molecular structure2.jpg|thumb|left|alt=A diagram of an opal's molecular structure, showing small lilac spheres packed in misaligned sheets on top of each other. The legend shows that one sphere has a diameter of 150–300 nanometres.|Precious opal consists of  a regular arrangement of [[silicon dioxide|silica]] [[Nanoparticle|nanospheres]] in closely packed planes (idealized diagram).]]
[[File:Coober Pedy Opal 2.jpg|thumb|left|alt=A small opal held side-on between someone's forefinger and thumb. A small sliver of brown rock is visible at the top, with a larger section of opal below it.|This precious rough opal from [[Coober Pedy]], [[South Australia]], displays nearly every color of the [[visible spectrum]].]]

Precious opal shows a variable interplay of internal colors, and though it is a mineraloid, it has an internal structure. At microscopic scales, precious opal is composed of silica spheres some {{cvt|150|-|300|nm}} in diameter in a hexagonal or cubic [[close-packing of equal spheres|close-packed]] [[crystal structure|lattice]]. It was shown by J. V. Sanders in the mid-1960s<ref>{{cite journal |last=Sanders|first=J.&nbsp;V. |date=1964 |title=Colour of precious opal |journal=[[Nature (journal)|Nature]] |volume=204 |issue=496 |pages=1151–1153 |doi=10.1038/2041151a0 |bibcode=1964Natur.204.1151S |s2cid=4169953}}</ref><ref>{{cite journal |last=Sanders|first=J.&nbsp;V. |year=1968 |title=Diffraction of light by opals |journal=Acta Crystallographica A |volume=24 |issue=4 |pages=427–434 |doi=10.1107/S0567739468000860 |bibcode=1968AcCrA..24..427S}}</ref> that these ordered silica spheres produce the internal colors by causing the [[interference (wave propagation)|interference]] and [[diffraction]] of light passing through the microstructure of the opal.<ref name=Klein>{{cite book |last1=Klein|first1=Cornelis |last2=Hurlbut|first2=Cornelius S. |date=1985 |title=Manual of Mineralogy |publisher=Wiley |edition=20th |isbn=978-0-471-80580-9 |url-access=registration |url=https://archive.org/details/manualofmineralo00klei}}</ref> The regularity of the sizes and the packing of these spheres is a prime determinant of the quality of precious opal. Where the distance between the regularly packed planes of spheres is around half the wavelength of a component of [[visible light]], the light of that wavelength may be subject to diffraction from the [[diffraction grating|grating]] created by the stacked planes. The colors that are observed are determined by the spacing between the planes and the orientation of planes with respect to the incident light. The process can be described by [[Bragg's law]] of diffraction.

Visible light cannot pass through large thicknesses of the opal. This is the basis of the optical [[band gap]] in a [[photonic crystal]].<ref>{{cite journal |last1=Astratov|first1=V.&nbsp;N. |year=1995 |title=Optical spectroscopy of opal matrices with CdS embedded in its pores: Quantum confinement and photonic band gap effects |journal=Il Nuovo Cimento D |volume=17 |issue=11–12 |pages=1349–1354 |last2=Bogomolov|first2=V.&nbsp;N. |last3=Kaplyanskii|first3=A.&nbsp;A. |last4=Prokofiev|first4=A.&nbsp;V. |last5=Samoilovich|first5=L.&nbsp;A. |last6=Samoilovich|first6=S.&nbsp;M. |last7=Vlasov|first7=Yu.&nbsp;A. |doi=10.1007/bf02457208 |bibcode=1995NCimD..17.1349A |s2cid=121167426}}</ref> In addition, microfractures may be filled with secondary silica and form thin [[lamella (materials)|lamellae]] inside the opal during its formation. The term ''[[opalescence]]'' is commonly used to describe this unique and beautiful phenomenon, which in gemology is termed ''[[iridescence|play of color]]''. In gemology, opalescence is applied to the hazy-milky-[[turbidity|turbid]] sheen of common or potch opal which does not show a play of color.{{clarify|reason=this sentence appears to contradict the previous one|date=April 2022}} Opalescence is a form of [[adularescence]].

For gemstone use, most opal is cut and polished to form a [[cabochon]]. "Natural" opal refers to polished stones consisting wholly of precious opal. Opals too thin to produce a "natural" opal may be combined with other materials to form "composite" gems. An opal [[doublet (lapidary)|doublet]] consists of a relatively thin layer of precious opal, backed by a layer of dark-colored material, most commonly ironstone, dark or black common opal (potch), onyx, or obsidian. The darker backing emphasizes the play of color and results in a more attractive display than a lighter potch. An opal triplet is similar to a doublet but has a third layer, a domed cap of clear [[quartz]] or plastic on the top. The cap takes a high polish and acts as a protective layer for the opal. The top layer also acts as a magnifier, to emphasize the play of color of the opal beneath, which is often an inferior specimen or an extremely thin section of precious opal. Triplet opals tend to have a more artificial appearance and are not classed as precious gemstones, but rather "composite" gemstones. Jewelry applications of precious opal can be somewhat limited by opal's sensitivity to heat due primarily to its relatively high water content and predisposition to scratching.<ref>{{cite web |author1=Dr. Joel Arem |author2=Donald Clark, CSM IMG |url=http://www.gemsociety.org/article/opal-jewelry-and-gemstone-information/ |title=Opal Value, Price, and Jewelry |website=Gemsociety.org |date=2015-06-23 |access-date=2016-11-22 |url-status=live |archive-url=https://web.archive.org/web/20161123060711/https://www.gemsociety.org/article/opal-jewelry-and-gemstone-information/ |archive-date=23 November 2016 |df=dmy-all }}</ref> Combined with modern techniques of polishing, a doublet opal can produce a similar effect to Natural black or boulder opal at a fraction of the price. Doublet opal also has the added benefit of having genuine opal as the top visible and touchable layer, unlike triplet opals.