Precious Opal Research Articles

Geoheritage of the Precious Opal Bearing Zone in Libanka Mining District (Slovakia) and Its Geotourism and Geoeducation Potential

The northern part of the Slanské Vrchy Mountains, in Slovakia, within the territory of Dubnik Opal Mines, is considered to be the historical source of opal, including the opal known to the Romans. Dubnik precious opal mineralization originating from post-magmatic hydrothermal fluids based on its values should be recognized as an area of global importance. Geological processes that are connected to Neogene volcanic activity took place in the territory of the Dubnik Opal Mines and resulted in the formation of various geological products and processes represented by polymetallic mineral formation, opal formation, antimonite formation, and mineralization of the supergene stage, as well as the stratovolcano rock environment, which are of geoheritage significance. Remnants of intensive mining activity that took place during the past 2000 years provide opportunities for visitors to explore, study, and appreciate this unique geological heritage of international importance. However, geo-interpretation aimed at the general public (the largest group of potential visitors from sustainable geotourism development) and research on (geo)tourism and/or geoheritage for tourism purposes, as discussed in this paper, are lacking.

Characterization of Natural Precious Opal Using Modern Spectroscopic Techniques in Ethiopia: The Case from Delanta, South Wollo

The purpose of this study was to conduct characterization of the Delanta natural opals obtained from different mining sites. Characteristics of opals mined from different geological locality were examined. Six opal samples were collected from the main mining sites found in different localities from Delanta district and characterized using Fourier infrared spectroscopy, scanning electron microscopy, X-ray diffraction, and inductively coupled plasma optical emission spectroscopy characterization techniques. The scanning electron microscope micrographs showed that all the samples have relatively semi-ordered (semi-crystalline) structural surface morphologies composed of tiny spherical particles. The performed X-ray diffraction analysis showed that the samples have main peaks at 20.0°, 21.72°, 36.9°, 44.0°, and 56.85° with 2 θ values between 10° and 60°. The five diffraction peaks are characteristic peak positions of opal-CT. The Fourier infrared spectroscopy data indicated that the prominent absorption bands for cristobalite at 620 and 520 cm−1 were absent from the spectrum. Instead, three bands in the low frequency region centered at 470, 791, and 1100 cm−1 were observed because of the absorptions of the silicate framework (Si-O) vibrations. red−1−1 Moreover,spectra feature of molecular water and silanol (Si-OH) groups with broadabsorption band centered around 3447 cm-1 andthe O-H stretching vibration of watermolecules and water bending vibration withan absorption band centered around 1634 cm-1were observed.. These values were consistently comparable with literature data. The inductively coupled plasma optical emission spectroscopy analysis showed that four main impurities (>500 parts per million) were Ca (∼1,750–4,730 parts per million), Al (∼1,990–4,319 parts per million, K (∼1,670–3,895 parts per million), and Na (∼595–3,723 parts per million). In general, this study revealed that all opal samples taken from six mining sites in Delanta district were found to be opal-CT as per the results of X-ray diffraction and Fourier infrared spectroscopy investigations.

Coherent poly propagation materials with 3-dimensional photonic control over visible light.

The purpose of the present research was to identify and examine materials demonstrating a previously undiscovered property of coherent poly propagation (CPP). The materials were amorphous silicates as natural precious opals. CPP enabled three-dimensional photonic control over mono and polychromatic visible light wavelengths. CPP caused coherent diffraction of incident poly and monochromatic light. Apart from the iconic play-of-color of precious opal, CPP specimens demonstrated diffractive photonic demultiplexing and/or upconversion and/or downconversion of incident light with strong photonic coherence such that the shape of the incident light source was propagated over three dimensions over multiple visible frequencies. CPP events manifested as each specimen was rocked under the incident light. Additionally, the specimens demonstrated atypical control over internally reflected and transmitted light. The specimens applied axial rotational symmetry over the incident light. Amorphous materials would be expected to exert no symmetry control. CPP and rotational properties occurred in isolation from exogenous thermal, photonic and electrical influences. Furthermore, several non-destructive analytical instruments were employed, such as: spectrophotometer, polariscope and refractometer. The analytical methods revealed unusual behaviors of these specimens. The application of materials demonstrating three-dimensional photonic control will have far-reaching implications for many industries, including: photonic wavelength demultiplexing, fiber optics, imaging, microscopy, projections, security, cryptography, computers and communications.

Paleomagnetic dating of ironstone nodules (‘nuts’) from the Yowah opal field, central southern Queensland

ABSTRACTTo constrain the age of Australian opal formation, we have undertaken a paleomagnetic study of oxidised ironstone ‘nuts’ from Yowah, Queensland. Following standard methods, we have calculated a mean direction of declination D = 191.4°, inclination I = 61.7° (α95 = 4.0°), indicating a paleomagnetic pole position at latitude λp = 71.3°S, longitude ϕp = 119.4°E (A95 = 5.3°). The direction comprises both normal and reverse polarities that fail a reversal test most probably owing to contamination by small recent/present-day components. The mean direction should not be significantly affected. A chi-square comparison with paleomagnetic poles for dated Cenozoic rocks in eastern Australia, poles derived from the Global Moving Hotspot Reference Frame and the Cenozoic pole path for North America, appropriately transferred to Australian coordinates, yields a mean age estimate of 35 ± 7 Ma, i.e. late Eocene to early Oligocene. This is interpreted as the age of the ironstone formation, which places a maximum age for the formation of precious Yowah opal. This result confirms and tightens the age for the nearby Canaway weathered profile.

The Precious Opal Deposit At Wegel Tena, Ethiopia: FormationViaSuccessive Pedogenesis Events

Abstract Since its discovery in 2008, the opal deposit in the Wegel Tena area of Ethiopia, located within an Oligocene ignimbritic trap, has become a major provider of precious opal. This study provides new field evidence and laboratory results that reinforce the pedogenetic model for gem opal formation. In the eight mines investigated, opal occurs in lenticular horizons within an unwelded ignimbrite bed. Moreover, several deposits are superimposed in the sequence, indicating that gem opal formation occurred several times in the same place. We observed that, in contrast to the other beds, the opal-bearing horizons are strongly weathered: volcanic glass is altered to smectite, rocks with a granular microstructure are apparent, and cavities are covered by illuvial clays. These features are typical of pedogenetic processes. Plant fossils in the opals reveal a well preserved cellular structure composed of chalcedony. These observations are consistent over all the mines, supporting the pedogenetic model for precious opal formation in the Wegel Tena area.

The ‘fire’ of opals

T he mineral opal, chemically a form of hydrated silica, is found on practically all continents, but mostly as an “opalescent”, milky–white, soft rock. However, in some parts of Australia small pieces of beautifully coloured gemstones, “precious opal”, are to be found embedded in a matrix of ordinary opal. What makes these quintessentially Australian gemstones sparkle with flecks of pure spectral colour (Fig.1)? Oddly enough, the answer to this question was a mystery to mineralogists for a long time until noted CSIRO electron microscopist John Sanders discovered the surprising answer as recently as the 1970s [1],[2]. Because of the spectral colours exhibited, the phenomenon of diffraction from periodic features was suspected to be the cause, but nobody could guess at the nature of such periodic structures until they were revealed by electron microscopy. It was surmised that the optical properties of precious opal, as distinct from the milky-white appearance of common opal that shows no ‘fire’, depends on the existence of orderly, regular arrays of optical discontinuities, spaced at repeat distances of the order of 150 to 350 nanometers, i.e., distances that correspond to half the wavelength of visible light. Chemically, opals are made of pure, transparent, hydrated silica, i.e., hydrated Silicon Dioxide. But what the electron microscope revealed was that the silica is in the form of tiny spheres, of the appropriate range of sizes, stacked in close-packed regular arrays, as may be seen in Fig. 2, just like atoms or molecules in crystalline substances. How are these little spherical objects formed is answered by noting that the solubility of silica in water increases markedly with temperature so that, upon cooling, silica is usually deposited as quartz crystals that are said to grow in what is called a hydrothermal process. Alternatively, in the presence of centres of nucleation, the silica can precipitate from saturated solutions in the form of amorphous clusters. These continue to grow as concentric spheres, which then fall through the solution and end up in interstitial cavities. In most cases, a poly-disperse range of sizes results, which when dried out results in a milky-white solid of ordinary, or so-called ‘potch’ opal. However, in rare cases, where the little spheres have a greater distance through which to fall, a gravitational separation can take place, where the larger spheres fall more quickly than the smaller ones and arrange themselves in layers upon layers of uniformly sized regions of hexagonal close-packed groups, like oranges in a crate. Hence the quasi-crystalline arrangement of precious opal, usually in small pieces consisting of separate small regions, is analogous to crystal grains. The opal ‘grains’ can vary in size, from a few millimeters, known as pin-fire opals, up to quite large ones in what is known as boulder opals. Because the individual silica spheres are completely transparent, pieces of opal show practically no colour when viewed in transmission. However, when viewed in reflection, strong diffraction colours are seen. These diffraction phenomena are not like those from a two-dimensional grating, such as is seen from the surface of a CD or DVD, and erroneously shown in illustrations in some popular articles. On the contrary, 3-dimensional diffraction

Water and hydrogen release from perlites and opal

The dehydration of two specimens of perlite from Palhaza (Hungary) and from Borovitza (Bulgaria) along with a specimen of Australian precious opal has been investigated by temperature controlled high-vacuum degassing experiments (DEGAS). Dehydration results in the loss of water over a wide range of temperatures due to the presence of both molecular and bound (silanol, Si–OH) water. A surprising observation is that hydrogen (H2) is also released. The hydrogen is observed to be released from both perlites and opal and both by diffusional processes and through the sudden explosive, bursting of inclusions which produce sharp spikes in the MS data. The origin of the hydrogen cannot be explained by a simple statistical distribution of the silanol species in the vitreous matrix; rather, it is more likely to be associated with an inverse-micellar decomposition of silanol species. Alternate sources of hydrogen such as meteoric waters or specific genetic origins based on the perlite or opal location and formation are discounted as the hydrogen release is found to be independent of CO2 and SO2 evolution.

Australian sedimentary opal-A and its associated minerals: Implications for natural silica sphere formation

The vast majority of precious opal on the world market comes from opal fields in the Great Artesian Basin of Australia pointing to very special prerequisites for amorphous silica to consolidate in a way that leads to the famous play-of-color. We analyzed 20 opal-A samples from the Andamooka (South Australia) and Yowah (Queensland) precious opal fields, using petrographic microscopy, XRPD, SEM, and EPMA to identify and characterize opaline silica, the mineral assemblage, and the host rock. Opal-A consists of submicrometer-sized silica spheres with an average diameter of 140-320 nm. The average diameter of monodisperse spheres is 140-290 nm with a relative standard deviation (RSD) of <6%. Polydisperse spheres show an average diameter of 160-320 nm with a RSD larger than 10%. This dichotomy in size is reflected by the Na/K ratio at both localities. Monodisperse spheres show values below 1.2 while polydisperse ones show a ratio larger than 3.0, whereas other contaminations with higher valence show no correlations at all. We therefore suggest that the jump in Na/K signals a fundamental change of pH and salinity of the silica-bearing mineralizing fluids. Judging from the pH stability of the host rock minerals with predominating alunite, kaolinite, illite and gypsum, and omnipresent barite and anatase we conclude that the dominant late-stage mineralization leading to precious opal happened at acidic pH. Our findings indicate that the host rocks and associated minerals are the key to unravel the complex history of opal-forming solutions. A quantitative opal classification based on sphere diameters and their variability, decoupled from gemological properties, is to be established.

Remarks on the age of precious opal in central Australia: reply to comment by Bruce Dickson

Opal fields abound with stories about opalised beer bottles, opalised fence posts, and precious opal growing in jars. As noted by Dickson (2013), the myth of the opalised bottle was busted recently...

Discussion on ‘Opalisation of the Great Artesian Basin (central Australia): an Australian story with a Martian twist’ by P. F. Rey (AJES 60, 291–314)

In his paper ‘Opalisation of the Great Artesian Basin (central Australia): an Australian story with a Martian twist,’ Rey (2013) proposes that the bulk of precious opal was formed during the period...

Origin of silica and fingerprinting of Australian sedimentary opals

Opal is Australia's national gemstone with a significant fraction of the global supply mined from highly weathered Cretaceous sedimentary rocks within the Great Artesian Basin. Surprisingly, relatively little is known about the petrography and trace elemental composition of opal and its host rocks and consequently about the source of silica that underpins its formation. Using laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) of precious and common opal from key opal mining areas in the Great Artesian Basin coupled with multivariate analyses of 59 detectable elements in opal, we show that a mining region from which an opal originates can be constrained by using a combination of Hf, Ba, Zr and Gd with a high degree of confidence. Likewise, precious opal can be distinguished from common (non-precious opal) using a combination of Bi, Ta, Sn and Ca as these particular elements are especially low in concentration in precious opal. Although the opal from the Great Artesian Basin is sedimentary, the Ba content of opals from the eastern part of the basin suggests a volcanic origin. The most likely source of Ba and hence of silica for these opals are feldspars, now altered to kaolinite, sourced as volcaniclastic sediment from the Cretaceous Whitsunday Volcanic Province that marked the rifting and breakup of eastern Gondwana. The alteration of detrital feldspars to kaolinite and their replacement by void-filling opal confirms that weathering has played a critical role in the formation of Australian opal. The opal host rocks are severely weathered with a chemical index of alteration (CIA) up to 92. For the majority of opals studied, the silica is most likely derived locally from the opal host rocks, which impart a unique elemental signature on the opal at any particular locality. Mintabie opal, however, has very low Zr/Hf ratio, which is decoupled from its host rock, suggesting that the silica source is different from all the other opals, or that the silica fluid has experienced intense trace element fractionation, or both. The combination of analytical and statistical methods used here provides a powerful tool for a wide range of provenance studies, not just gemstones, where relationships between a large number of major and trace elements are difficult to unravel.

Opalisation of the Great Artesian Basin (central Australia): an Australian story with a Martian twist

This paper exposes the unique set of attributes explaining why precious opal has formed in such abundance in central Australia, and almost nowhere else on Earth. The Early Cretaceous history of the Great Artesian Basin is that of a high-latitude flexural foreland basin associated with a Cordillera Orogen built along the Pacific margin of Gondwana. The basin, flooded by the Eromanga Sea, acted as a sink for volcaniclastic sediments eroded from the Cordillera's volcanic arc. The Eromanga Sea was shallow, cold, poorly connected to the open ocean, muddy and stagnant, which explains the absence of significant carbonates. Iron-rich and organic matter-rich sediments contributed to the development of an anoxic sub-seafloor in which anaerobic, pyrite-producing bacteria thrived. Rich in pyrite, ferrous iron, feldspar, volcanic fragments and volcanic ash, Lower Cretaceous lithologies have an exceptionally large acidification potential and pH neutralisation capacity. This makes Lower Cretaceous lithologies particularly reactive to oxidative weathering. From 97 to 60 Ma, Australia remained at high latitude, and a protracted period of uplift, erosion, denudation and crustal cooling unfolded. It is possibly during this period that the bulk of precious opal was formed via acidic oxidative weathering. When uplift stopped at ca 60 Ma, the opalised redox front was preserved by the widespread deposition of a veneer of Cenozoic sediments. On Earth, regional acidic weathering is rare. Interestingly, acidic oxidative weathering has been documented at the surface of Mars, which shares an intriguing set of attributes with the Great Artesian Basin including: (i) volcaniclastic lithologies; (ii) absence of significant carbonate; (iii) similar secondary assemblages including opaline silica; (iv) similar acidic oxidative weathering driven by very similar surface drying out; and, not surprisingly, (v) the same colour. This suggests that the Australian red centre could well be the best regional terrestrial analogue for the surface of the red planet.

Towards a predictive model for opal exploration using a spatio-temporal data mining approach

Australia produces over 90% of the world's precious opal from highly weathered Cretaceous sedimentary rocks within the Great Artesian Basin. Since opal was first discovered around 1870 until the present day, opal mining has been carried out by private operators working a claim no larger than 50 × 50 m, usually in the direct vicinity of areas that have yielded precious opal in the past. Currently there is no formal exploration model for opal and its formation in the geological environment is poorly understood. Here we make the first systematic attempt to formulate a predictive model for opal exploration using a powerful data mining approach, which considers almost the entire Great Artesian Basin as a potential reservoir for precious opal. Our methodology uses all known locations where opal has been mined to date. Its formation and preservation in weathered Cretaceous host rocks is evaluated by a joint analysis of large digital data sets that include topography, regional geology, regolith and soil type, radiometric data and depositional environments through time. By combining these data sets as layers enabling spatio-temporal data mining using the GPlates PaleoGIS software, we produce the first opal prospectivity map for the Great Artesian Basin. Our approach reduces the entire area of the Great Artesian Basin to a mere 6% that is deemed to be prospective for opal exploration. It successfully identifies two known major opal fields (Mintabie and Lambina) that were not included as part of the classification dataset owing to lack of documentation regarding opal mine locations, and it significantly expands the prospective areas around known opal fields particularly in the vicinity of Coober Pedy in South Australia and in the northern and southern sectors of the Eromanga Basin in Queensland. The combined characteristics of these areas also provide a basis for future work aimed at improving our understanding of opal formation.

Interactions of low-power photons with natural opals—PBG materials, photonic control, natural metamaterials, spontaneous laser emissions, and band-gap boundary responses

Abstract One overall goal of this research was to examine types of naturally-occurring opals that exhibit photonic control to learn about previously-unknown properties of naturally occurring photonic control that may be developed for broader applications. Three different photon sources were applied consecutively to three different types of natural, flawless, gem-quality precious opals. Two photon sources were lasers (green and red) and one was simulated daylight tungsten white. As each type of precious opal was exposed to each of the photon sources, the respective refractions, reflections, and transmissions were studied. This research is the first to show that applying various pleochroic and laser photon sources to these types of opals revealed significant information regarding naturally occurring photonic control, metamaterials, spontaneous laser emissions, and microspheroid cluster (inter-PBG zone) boundary effects. Plus, minimizing ambient light and the use of low power photon sources were critical to observing the properties regarding this photonic materials research. This research yielded information applicable to the development of materials to advance applications and devices of photonics, phononics, optoelectronics, nanomaterials, and metamaterials.

Model Mineralisasi Pembentukan Opal Banten

DOI: 10.17014/ijog.v5i3.100Opal is a beautiful precious gemstone that is equal or more valuable than diamond. In Indonesia, precious opal is found at the Lebak Regency, Province of Banten. Banten’s opal widely has been recognized due to its beautiful opalescence. This paper is a review of the last research; preliminary study of Banten’s opal, characteristics of opal-CT and determining of opal type from geochemical data, added by new data to compile concept and to make mineralization model. In order to fulfill these targets, field geology research and analysis of mineralog/gemology, petrography, X-RD, and major and trace element geochemistry have been done. The Banten’s opal is opal-CT showing opalescence (play of colour), weathering, and leaching silica from volcanic glass by dark grey claystone hosted. Mineralization model is divided into three periods; at Early Pliocene volcanic clastic sediments rich in volcanic glass occured as fluvial sediments. Afterwards, at Late Pliocene - Pleistocene folding, weathering and leaching of silica took place. Intensive jointing, faulting, and folding quickened weathering and leaching processes to formed opal at limb of anticline through Holocene. The prospecting area of Banten’s opal is in tuff unit with intercalation of conglomerate or pumiceous breccia, at limb of anticline. The host rock of opal is dark grey claystone which underlies polimict conglomerate/pebbly sandstone sequence with cross stratification, imbricated, and erossional stucture; more than 8 m deep.

Self-assembly in the growth of precious opal

It is proposed that primary nucleation of amorphous microspherulites of hydrated silica in natural proto-precious-opal can be followed by a long range superlattice ordering process by means of electrostatic self-assembly. Necessary conditions in the thermodynamics are a high surface charge density on microspherulite surfaces, a long Debye length and an appropriate number density of nucleation centres. A further chemical requirement is a high alkaline environmental pH ∼9–10. It is also proposed that the characteristic concentric spherical shell-like structure of spherulites, centred on primary nuclei, are due to sequential deposition of intrinsic salts, which precipitate out when the corresponding solubility limits in the liquid are successively exceeded. It can be that the better-known sedimentation of microspherulites under gravity only plays part in the final stabilization period of overall growth.

Spherulites and spherulitic tactoids of anthraxolite

A pebble of anthraxolite found in the basin of the Kendei River, a tributary of the Lena River, was examined under polarization metallographic and electron microscopes to get an insight into their structure and ontogeny. A polished section of the mineral demonstrates evident anisotropy in polarized reflected light and finely sculptured microrelief of growth and induction surfaces of spherulites on the fracture (fractograms). The morphogenetic inspection and study of ontogenetic signs of origination, growth, phase transformations, and other changes of anthraxolite revealed a growth mechanism typical of spherulitic dendrites owing to low-angle splitting of radial subindividuals. Layered gravity structures (‘mineralogical levels’) have been found in crystallization voids. They are composed of an ordered even-grained aggregate of spherical crystals of anthraxolite with the tactoid texture of precious opal. The tactoid mineralogical levels of anthraxolite are characterized by the block structure of microspherulite aggregates, signs of deformation, twinning and geometrical selection of blocks, and elements of recrystallization. Growth-oriented zones of regular nets with endotaxial inclusions of graphite inside the cells have been observed on induction surfaces in spherulitic dendrites. Their appearance is considered an indicator of phase transformations. Supramolecular structures and their effect on the morphology of anthraxolite are discussed in short.

The nanostructure of fire opal

Fire opal, a transparent orange variety of opal, which does not diffract visible light, is built from the random accumulation of granular particles of hydrated silica, about 20 nm in size. This opal variety does not present the structure most commonly associated with precious opal, that is, a regular three-dimensional network of amorphous silica spheres about 200 nm in diameter. About 60 samples (from Mexico, Brazil, Kazakhstan, Ethiopia, Tanzania, Slovakia and USA) were documented using scanning electron and atomic force microscopy. This work demonstrates that nanograins are the elementary building blocks of this variety of opal.

Synthesis of precious opal in a hydrothermal solution

The nucleation of orthosilicic acid and the growth of colloidal silica particles in hydrothermal solutions are investigated. The results obtained are compared with the data on the formation of supramolecular matrices of precious opal. Ordered supramolecular structures are experimentally grown in a model solution through the precipitation of colloidal silica particles synthesized by hydrolysis of tetraethoxysilane in an organic solvent. The type of packing of spherical particles in the supramolecular structure is studied as a function of the component concentration, particle size, and pH and temperature of the model solution. The experimental data can be used to reveal the mechanism of formation of precious opal and to design methods for preparing three-dimensional nanocomposites.

Nanostructured polymer gels

AbstractA brief review of our recent work on polymer gel opals is given in memory of late Professor Tanaka. The central idea is to first synthesize monodisperse polymer gel nanoparticles, then self‐assemble them into a 3D network, and eventually covalently bond them. The covalent bonding contributes to the structural stability, while self‐assembly provides them with crystal structures that diffract light, resulting in colors. N‐isopropylacrylamide (NIPA) gel has been used as a model system for this study. The nanostructured NIPA gel, which contain up to 97 wt% water, displays a striking iridescence like precious opal but are soft and flexible like gelatin.