A rich seam of iridescent opal encased in matrix
(repeating unit)
Hydrated silica. SiO2·nH2O
Crystal systemAmorphous[1]
ColorColorless, white, yellow, red, orange, green, brown, black, blue, pink
Crystal habitIrregular veins, in masses, in nodules
FractureConchoidal to uneven[1]
Mohs scale hardness5.5–6[1]
LusterSubvitreous to waxy[1]
Diaphaneityopaque, translucent, transparent
Specific gravity2.15+0.08
Polish lusterVitreous to resinous[1]
Optical propertiesSingle refractive, often anomalous double refractive due to strain[1]
Refractive index1.450+0.020

Mexican opal may read as low as 1.37, but typically reads 1.42–1.43[1]
Ultraviolet fluorescenceblack or white body color: inert to white to moderate light blue, green, or yellow in long and short wave, may also phosphoresce, common opal: inert to strong green or yellowish green in long and short wave, may phosphoresce; fire opal: inert to moderate greenish brown in long and short wave, may phosphoresce[1]
Absorption spectragreen stones: 660 nm, 470 nm cutoff[1]
Diagnostic featuresdarkening upon heating
Solubilityhot salt water, bases, methanol, humic acid, hydrofluoric acid
Common Rough Opal

Opal is a hydrated amorphous form of silica (SiO2·nH2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Because of its amorphous character, it is classed as a mineraloid, unlike crystalline forms of silica, which are classed as minerals. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt. Opal is the national gemstone of Australia.[4]

There are two broad classes of opal: precious and common. Precious opal displays play-of-color (iridescence), common opal does not.[5] Play-of-color is defined as "a pseudochromatic optical effect resulting in flashes of colored light from certain minerals, as they are turned in white light."[6] The internal structure of precious opal causes it to diffract light, resulting in play-of-color. Depending on the conditions in which it formed, opal may be transparent, translucent or opaque and the background color may be white, black or nearly any color of the visual spectrum. Black opal is considered to be the rarest, whereas white, gray and green are the most common.

Precious opal

Precious opal consists of spheres of silicon dioxide molecules arranged in regular, closely packed planes. (Idealized diagram)
Multicolor rough crystal opal from Coober Pedy, South Australia, expressing nearly every color of the visible spectrum
Precious opal replacing ichthyosaur backbone; display specimen, South Australian Museum

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 150 to 300 nm in diameter in a hexagonal or cubic close-packed lattice. It was shown by J. V. Sanders in the mid-1960s[7][8] that these ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal.[9] The regularity of the sizes and the packing of these spheres determines 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 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. The notion that opals are photonic crystals for visible light was expressed in 1995 by Vasily Astratov's group.[10] In addition, microfractures may be filled with secondary silica and form thin lamellae inside the opal during solidification. The term opalescence is commonly used to describe this unique and beautiful phenomenon, which in gemology is termed play of color. In gemology, opalescence is applied to the hazy-milky-turbid sheen of common or potch opal which does not show a play of color. Opalescence is a form of adularescence.

For gemstone use, most opal is cut and polished to form a cabochon. "Solid" opal refers to polished stones consisting wholly of precious opal. Opals too thin to produce a "solid" may be combined with other materials to form attractive gems. An opal 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 of lower quality. Triplet opals therefore have a more artificial appearance, and are not classed as precious opal. 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.[11] Combined with modern techniques of polishing, doublet opal produces a similar effect to 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.

Other Languages
العربية: أوبال
беларуская: Апал
български: Опал
bosanski: Opal
català: Òpal
Чӑвашла: Опал
čeština: Opál
dansk: Opal
Deutsch: Opal
eesti: Opaal
español: Ópalo
Esperanto: Opalo
euskara: Opalo
français: Opale
Gaeilge: Ópal
galego: Ópalo
한국어: 단백석
հայերեն: Օպալ
हिन्दी: ओपल
hrvatski: Opal
Ido: Opalo
Ilokano: Ópalo
Bahasa Indonesia: Opal
íslenska: Ópall
italiano: Opale
עברית: אופאל
қазақша: Опал
Кыргызча: Опал
lietuvių: Opalas
magyar: Opál
مصرى: اوبال
Nederlands: Opaal
日本語: オパール
norsk: Opal
norsk nynorsk: Opal
پنجابی: اوپل
polski: Opal
português: Opala
română: Opal
русский: Опал
саха тыла: Опаал
Simple English: Opal
slovenčina: Opál
slovenščina: Opal
српски / srpski: Опал
srpskohrvatski / српскохрватски: Opal
suomi: Opaali
svenska: Opal
Tagalog: Opalo
ไทย: โอปอล
Türkçe: Opal
українська: Опал
Tiếng Việt: Opan
中文: 蛋白石