Black opal

Black opal is directly mined in New South Wales, Australia, making it the rarest type of opal. Having a darker composition, this gemstone can be in a variety of colours.

The main deposits of the rare black opal are near Lightning Ridge (New South Wales in Australia), particularly in the Coocoran , Grawin and Glengarry opal fields. Other larger deposits in Australia are Mintabie in South Australia; smaller deposits have also been found in Coober Pedy and Andamooka. More recent finds come from the Stayish Mine in Ethiopia. In addition to black opal, a distinction is made between black crystal opal, which is transparent to a certain degree. There is also dark opal, which remains dark even in transmitted light, and light grey opal. These varieties are mainly found near Mintabie; they are found wherever black opals occur.

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%. Due to the amorphous (chemical) physical structure, it is classified as a mineraloid, unlike crystalline forms of silica, which are considered 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.

The name opal is believed to be derived from the Sanskrit word upala (उपल), which means 'jewel', and later the Greek derivative opállios (ὀπάλλιος).

There are two broad classes of opal: precious and common. Precious opal displays play-of-color (iridescence); common opal does not. Play-of-color is defined as "a pseudo chromatic optical effect resulting in flashes of colored light from certain minerals, as they are turned in white light." 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 the rarest, while white, gray, and green opals are the most common.

Opal

General

Category Mineraloid

Formula Hydrated silica. SiO2•nH2O

IMA symbol Opl

Crystal system Amorphous

Identification

Color Colorless, white, yellow, red, orange, green, brown, black, blue, pink

Crystal habit Irregular veins, in masses, in nodules

Cleavage None

Fracture Conchoidal to uneven

Mohs scale hardness 5.5–6

Luster Subvitreous to waxy

Streak White

Diaphaneity opaque, translucent, transparent

Specific gravity 2.15+0.08

−0.90

Density 2.09 g/cm3

Polish luster Vitreous to resinous

Optical properties Single refractive, often anomalous double refractive due to strain

Refractive index 1.450+0.020

−0.080

Mexican opal may read as low as 1.37, but typically reads 1.42–1.43

Birefringence none

Pleochroism None

Ultraviolet fluorescence black 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

Absorption spectra green stones: 660 nm, 470 nm cutoff

Diagnostic features darkening upon heating

Solubility hot salt water, bases, methanol, humic acid, hydrofluoric acid

Etymology

The word 'opal' is adapted from the Latin term opalus. The origin of this word in turn is a matter of debate, but most modern references suggest it is adapted from the Sanskrit word úpala meaning ‘precious stone’.

As references to the gem are made by Pliny the Elder, one theory attributed the name's origin to Roman mythology: to have been adapted from Ops, the wife of Saturn, and goddess of fertility. (The portion of Saturnalia devoted to Ops was "Opalia", similar to opalus.)

Another common claim was that the term was adapted from the Ancient Greek word, opallios. This word has two meanings, one is related to "seeing" and forms the basis of the English words like "opaque"; the other is "other" as in "alias" and "alter". It is claimed that opalus combined these uses, meaning "to see a change in color". However, historians have noted the first appearances of opallios do not occur until after the Romans had taken over the Greek states in 180 BC and they had previously used the term paederos.

However, the argument for the Sanskrit origin is strong. The term first appears in Roman references around 250 BC, at a time when the opal was valued above all other gems. The opals were supplied by traders from the Bosporus, who claimed the gems were being supplied from India. Before this, the stone was referred to by a variety of names, but these fell from use after 250 BC.

Classification

In the outdated 8th edition of the mineral classification according to Strunz, the opal belonged to the mineral class of “oxides and hydroxides” and there to the section “ MO 2 and related compounds ”, where it formed the “Lechatelierite-Opal Group” with the system number IV/D.01c together with lechatelierite, which was previously considered to be the mineral in question.

In the Lapis mineral catalog by Stefan Weiß, last revised and updated in 2018, which is still based on this old form of the systematics by Karl Hugo Strunz, the mineral was given the system and mineral number IV/D.01-080. In the " Lapis systematics ", this also corresponds to the section " Oxides with a molar ratio of metal to oxygen = 1:2 (MO 2 and related compounds) ", where opal, together with bosoite, chibaite, coesite, cristobalite, lechatelierite, melanophlogite, moganite, quartz, seifertite, stishovite, and tridymite, forms the "Quartz series" with the system number IV/D.01. 

The 9th edition of Strunz's mineral classification, last updated in 2009 by the International Mineralogical Association (IMA), also classifies opal in the class of "oxides and hydroxides" and there in the division "metal: oxygen = 1: 2 and comparable". However, this is further subdivided according to the relative size of the cations involved and the relationship of the minerals or the crystal structure, so that the mineral can be found in the subdivision " With small cations: Silicic acid family " according to its composition, where it forms the "opal group" with the system number 4.DA.10 only together with tridymite.

In contrast to Strunz's classification, the Dana classification of minerals, which is predominantly used in English-speaking countries, classifies opal in the class of "silicates" and within the division of " tectosilicate minerals." Here, it is the only member of the unnamed group 75.02.01 within the subdivision " tectosilicates: tetrahedral Si lattice, SiO 2 with H 2 O and organic molecules."

Characteristics

Chemical and general physical properties

As one of the few amorphous minerals, opal consists of hydrated silica gel with the general chemical composition SiO 2 •nH 2 O. The water content is usually between 4 and 9 percent, but can reach about 20 percent. Opals can be dissolved by hydrofluoric acid and potassium hydroxide. Opal does not melt when heated over an open flame, but becomes dull and crackles.

Local atomic structure

The lattice of spheres of opal that cause interference with light is several hundred times larger than the fundamental structure of crystalline silica. As a mineraloid, no unit cell describes the structure of opal. Nevertheless, opals can be roughly divided into those that show no signs of crystalline order (amorphous opal) and those that show signs of the beginning of crystalline order, commonly termed cryptocrystalline or microcrystalline opal. Dehydration experiments and infrared spectroscopy have shown that most of the H2O in the formula of SiO2•nH2O of opals is present in the familiar form of clusters of molecular water. Isolated water molecules, and silanols, structures such as SiOH, generally form a lesser proportion of the total and can reside near the surface or in defects inside the opal.

The structure of low-pressure polymorphs of anhydrous silica consists of frameworks of fully corner bonded tetrahedra of SiO4. The higher temperature polymorphs of silica cristobalite and tridymite are frequently the first to crystallize from amorphous anhydrous silica, and the local structures of microcrystalline opals also appear to be closer to that of cristobalite and tridymite than to quartz. The structures of tridymite and cristobalite are closely related and can be described as hexagonal and cubic close-packed layers. It is therefore possible to have intermediate structures in which the layers are not regularly stacked.

Microcrystalline opal

Microcrystalline opal or Opal-CT has been interpreted as consisting of clusters of stacked cristobalite and tridymite over very short length scales. The spheres of opal in microcrystalline opal are themselves made up of tiny nanocrystalline blades of cristobalite and tridymite. Microcrystalline opal has occasionally been further subdivided in the literature. Water content may be as high as 10 wt%. Opal-CT, also called lussatine or lussatite, is interpreted as consisting of localized order of α-cristobalite with a lot of stacking disorder. Typical water content is about 1.5 wt%.

Noncrystalline opal

Two broad categories of noncrystalline opals, sometimes just referred to as "opal-A" ("A" stands for "amorphous"), have been proposed. The first of these is opal-AG consisting of aggregated spheres of silica, with water filling the space in between. Precious opal and potch opal are generally varieties of this, the difference being in the regularity of the sizes of the spheres and their packing. The second "opal-A" is opal-AN or water-containing amorphous silica-glass. Hyalite is another name for this.

Noncrystalline silica in siliceous sediments is reported to gradually transform to opal-CT and then opal-C as a result of diagenesis, due to the increasing overburden pressure in sedimentary rocks, as some of the stacking disorder is removed.

Opal surface chemical groups

The surface of opal in contact with water is covered by siloxane bonds (≡Si–O–Si≡) and silanol groups (≡Si–OH). This makes the opal surface very hydrophilic and capable of forming numerous hydrogen bonds.

Optical properties

Many opals are used as gemstones due to their mottled, iridescent coloration. This process, called opalescence, occurs primarily in precious opals. It is caused by the reflection and interference of light rays between the silica gel spheres, which are between 150 and 400 nanometers in size. In precious opals, these silica gel spheres are approximately the same size and are arranged in a regular, densely packed arrangement. 

The similar-sounding term opalescence refers to a milky-bluish optical effect of common opals, similar to pearlescent luster. 

The base color of the opals must be distinguished from the play of colors. Common colors are black, white, gray, blue, green, and orange. The base color is independent of the play of colors and depends, among other things, on the chemical composition of the rocks in which the opals were formed. Chemical elements contained in the surrounding rock, such as iron, cobalt, copper, nickel, and silver, also have an influence. 

Precious opal 

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 150–300 nm (5.9×10−6–1.18×10−5 in) in diameter in a hexagonal or cubic close-packed lattice. It was shown by J. V. Sanders in the mid-1960s that these ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal. 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 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. In addition, microfractures may be filled with secondary silica and form thin 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 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. "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 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. 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.

A special type of opal is found in Queensland, Australia, and is collectively known as boulder opal. There are boulder matrix opals, which are bonded to the limonitic rock, and Yowah nut opals, which are enclosed in an iron-containing shell. Light, dark, and black boulder opals have the corresponding play of colors in the rock. There is also boulder split, an opal split along the grain with an identical color pattern.

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