What do we know about opal?
A lot of research on opal has been done over the past decades – some even over a hundred years ago. This page is a collection of research publications and field notes about Australian opal. If you want to learn more about opal – dig in!
Publications are divided into categories by their topic and year.
Opal as a Gemstone
Authors: Neville J. Curtis , Jason R. Gascooke , Martin R. Johnston and Allan Pring
Published in: Minerals 2019, 9(5), 299
Our examination of over 230 worldwide opal samples shows that X-ray diffraction (XRD) remains the best primary method for delineation and classification of opal-A, opal-CT and opal-C, though we found that mid-range infra-red spectroscopy provides an acceptable alternative. Raman, infra-red and nuclear magnetic resonance spectroscopy may also provide additional information to assist in classification and provenance. The corpus of results indicated that the opal-CT group covers a range of structural states and will benefit from further multi-technique analysis. At the one end are the opal-CTs that provide a simple XRD pattern (“simple” opal-CT) that includes Ethiopian play-of-colour samples, which are not opal-A. At the other end of the range are those opal-CTs that give a complex XRD pattern (“complex” opal-CT). The majority of opal-CT samples fall at this end of the range, though some show play-of-colour. Raman spectra provide some correlation. Specimens from new opal finds were examined. Those from Ethiopia, Kazakhstan, Madagascar, Peru, Tanzania and Turkey all proved to be opal-CT. Of the three specimens examined from Indonesian localities, one proved to be opal-A, while a second sample and the play-of-colour opal from West Java was a “simple” Opal-CT. Evidence for two transitional types having characteristics of opal-A and opal-CT, and “simple” opal-CT and opal-C are presented.
Keywords: opal; hyalite; silica; X-ray diffraction; Raman; Infrared; 29Si nuclear magnetic resonance; SEM; provenance
Authors: Gaillou, E.
Published in: Conference paper: Thirteenth Annual Sinkankas Symposium — Opal
Opal is one of the most fascinating gems, especially in the precious form that displays the spectacular rainbow flashes. Its appearance constantly changes when one looks at it, and we cannot really qualify its colors and grasp its essence, giving opal a unique, mysterious aspect.
Opal has a nomenclature of its own, depending on who is examining it: the mineralogist, the geologist, the gemologist, or the jeweler. We hear terms such as hyalite, jelly, noble, precious, fire, harlequin, black opal, and many more. These qualifiers can become confusing, but we will see that most of them refer to the opal’s transparency, its body color, or the presence (or absence) of the rainbow flashes referred to as play-of-color. To better understand all of these features, it is necessary to explore opal’s formation mode and its internal structure, subjects that will be developed here, and to examine the rea- sons for the many body colors opals can display.
Authors: Wang, Dadong & Bischof, Leanne & Lagerstrom, Ryan & Hilsenstein, Volker & Hornabrook, Angus & Hornabrook, Graham
Published in: IEEE Transactions on Systems, Man, and Cybernetics: Systems. 46. 1-1
Quantitative grading of opals is a challenging task even for skilled opal assessors. Current opal evaluation practices are highly subjective due to the complexities of opal assessment and the limitations of human visual observation. In this paper, we present a novel machine vision system for the automated grading of opals-the gemological digital analyzer (GDA). The grading is based on statistical machine learning with multiple characteristics extracted from opal images. The assessment work-flow includes calibration, opal image capture, image analysis, and opal classification and grading. Experimental results show that the GDA-based grading is more consistent and objective compared with the manual evaluations conducted by the skilled opal assessors.
Authors: Theodore Grussing
Published in: Gems & Gemology: The Quarterly Journal of the Gemological Institute of America, Vol. LII
Cutting large gem-quality opal rough poses special challenges not encountered when working with smaller pieces. The author explains the considerations in cutting a 3,019 ct piece of gem-quality white opal that was mined from the Olympic Field in Coober Pedy, South Australia, during the 1950s. Through careful analysis and planning, he was able to extract a single finished gem weighing 1,040 ct, with play-of-color across the entire surface. Named the Molly Stone, it is one of the largest fine gem opals ever cut. This article describes the unique factors involved in maximizing its size and play-of-color.
Authors: Don Emerson
Published in: Preview, 2016:185, 37-45
In this article the writer continues a quite subjective and idiosyncratic ramble through the mineral kingdom’s garden of gem showpieces (Editor’s note: See Preview 173 and 179 for other articles by Don Emerson in this vein).
For the novelist and psychopharmacological guru Aldous Huxley (1956), gemstones were the manifestation of a heightened mystical experience promising an environment:
of curved reflections, of softly lustrous glazes, of sleek and smooth surfaces. In a word, the beauty transports the beholder, because it reminds him, obscurely or explicitly, of the preternatural lights and colour of the Other World.
And none more so than the opal, the subject of this article. Over the ages gem opal has always been desired for jewellery and, as the queen of gems, regarded as an eminently collectible stone.
Authors: Smallwood, A. G., Thomas, P. S., & Ray, A. S
Published in: Journal of the Australian Ceramic Society, 44(2), 17–22.
Precious opal is Australia’s national gemstone, with Australian opal fields providing 90% of world production. The sedimentary geological environment, associated with Cretaceous sediments of the Great Artesian Basin, is the source of most precious opals in Australia. The deposit of precious opal at Tintenbar in northern New South Wales is the only known commercial occurrence of precious opal in volcanic environment in Australia. Differences in silica structure of opal previously classified by x-ray diffraction (XRD) in the 1960’s by Jones and Segnit identified three types of opal structure – amorphous opal-A, opal- CT with a poorly crystalline intergrowth cristobalite and tridymite and opal-C showing the cristobalite structure. Recent papers have suggested that all precious opal from a sedimentary environment is Opal-A, and all precious opal from the volcanic environment is opal-CT. This paper examines the differences between sedimentary precious opals from Coober Pedy, South Australia, and volcanic precious opal from Tintenbar, NSW using XRD, scanning electron microscopy and thermal analysis.
Authors: E. Gaillou, Aurélien Delaunay, B. Rondeau, Martine Bouhnik-Le Coz, E. Fritsch, et al
Published in: Ore Geology Reviews, Elsevier, 2008, 34 (1-2),pp.113-126.
Seventy-seven gem opals from ten countries were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) through a dilution process, in order to establish the nature of the impurities. The results are correlated to the mode of formation and physical properties and are instrumental in establishing the geographical origin of a gem opal. The geochemistry of an opal is shown to be dependant mostly on the host rock, at least for examples from Mexico and Brazil, even if modified by weathering processes. In order of decreasing concentration, the main impurities present are Al, Ca, Fe, K, Na, and Mg (more than 500 ppm). Other noticeable elements in lesser amounts are Ba, followed by Zr, Sr, Rb, U, and Pb. For the first time, geochemistry helps to discriminate some varieties of opals. The Ba content, as well as the chondrite-normalized REE pattern, are the keys to separating sedimentary opals (Ba > 110 ppm, Eu and Ce anomalies) from volcanic opals (Ba < 110 ppm, no Eu or Ce anomaly). The Ca content, and to a lesser extent that of Mg, Al, K and Nb, helps to distinguish gem opals from different volcanic environments. The limited range of concentrations for all elements in precious (play-of-color) compared to common opals, indicates that this variety must have very specific, or more restricted, conditions of formation. We tentatively interpreted the presence of impurities in terms of crystallochemistry, even if opal is a poorly crystallized or amorphous material. The main replacement is the substitution of Si4+ by Al3+ and Fe3+. The induced charge imbalance is compensated chiefly by Ca2+, Mg2+, Mn2+, Ba2+, K+, and Na+. In terms of origin of color, greater concentrations of iron induce darker colors (from yellow to “chocolate brown”). This element inhibits luminescence for concentrations above 1000 ppm, whereas already a low content in U (≤ 1 ppm) induces a green luminescence.
Keywords: Opal, Chemical composition, Trace element analysis, Genesis, ICP-MS
Authors: Smallwood, A & Thomas, Paul & Ray, Abhi
Published in: Australasian Institute of Mining and Metallurgy Publication Series.
The characterisation of the surface area and porosity of opals derived from Tintenbar, a volcanic environment, and Lightning Ridge, a sedimentary environment, using nitrogen gas adsorption at -196 o C is reported. Both opal types were found to have relatively low surface areas and displayed little porosity. The low surface areas observed is indicative of the ability of silica to infill voids and interstices. Thermogravimetric analysis of the samples before and after degassing was carried out to determine the amount of water removed by the degassing process. Negligible difference was found in the water content before and after degassing in the case of the Lightning Ridge sedimentary opal, while the Tintenbar volcanic opal was found to have lost more that 60% of its water during the degassing process. These differences were ascribed to the differences in the silica structure of the opals with the Lightning Ridge opal having a more dense cage structure which traps the molecular water while a more open structure is postulated for the Tintenbar opal allowing the water to be relatively easily removed.
Keywords: Opal, nitrogen adsorption, thermogravimetric analysis, volcanic, sedimentary
Authors: Gaillou, E., Fritsch, E., Aguilar-Reyes, B., Rondeau, B., Post, J., Barreau, A., & Ostroumov, M.
Published in: American Mineralogist, 93(11–12), 1865–1873.
The microstructure of nearly 200 common gem opal-A and opal-CT samples from worldwide localities was investigated using scanning electron microscopy (SEM). These opals do not show play-of-color, but are valued in the gem market for their intrinsic body color. Common opal-AG and opal-CT are primarily built from nanograins that average ~25 nm in diameter. Only opal-AN has a texture similar to that of glass. In opal-AG, nanograins arrange into spheres that have successive concentric layers, or in some cases, radial structures. Common opal does not diffract light because its spheres exhibit a range of sizes, are imperfectly shaped, are too large or too small, or are not well ordered. Opal-AG spheres are typically cemented by non-ordered nanograins, which likely result from late stage fluid deposition. In opal-CT, nanograins have different degrees of ordering, ranging from none (aggregation of individual nanograins), to an intermediate stage in which they form tablets or platelets, to the formation of lepispheres. When the structure is built of lepispheres, they are generally cemented by non-ordered nanograins. The degree of nanograin ordering may depend on the growth or deposition rate imposed by the properties of the gel from which opal settles, presumably, fast for non-ordered nanograin structures in opal-CT to slow for the concentric arrangement of nanograins in the spheres of opal-AG.
Authors: Sanders, J. V.
Published in: Philosophical Magazine A: Physics of Condensed Matter, Structure, Defects and Mechanical Properties, 42(6), 705–720.
An unusual sample of gem opal has been found to contain silica spheres of two different sizes, mixed in various proportions. Electron micrographs show extensive areas of long-range order. The structures of two different ordered phases, corresponding to compounds AB2 and AB13, have been deduced from the micrographs. Their calculated densities are found to be greater than that of the two separate components in a close-packed regular arrangement.
Authors: Jones, J. B., & Segnit, E. R.
Published in: Journal of the Geological Society of Australia, 18(1), 57–67.
Natural hydrous silicas may be subdivided into three well‐defined structural groups—opal‐C (well‐ordered α‐cristobalite), opal‐CT (disordered a‐cristobalite, a‐tridymite) and opal‐A (highly disordered, near amorphous). Lussatite from the original locality is identical with opal‐CT and thus appears to be a legitimate term for this class of opal. Although the prime criterion used is the nature of the X‐ray diffraction pattern, supplementary information from infra‐red absorption, dilatometer and thermal techniques supports the three‐fold classification.
Authors: Segnit, E. R., Stevens, T. J., & Jones, J. B.
Published in: Journal of the Geological Society of Australia, 12(2), 211–226
The occurrence of water in natural opaline silicas has been studied by differential thermal analysis, thermogravimetric analysis, infra‐red analysis and nuclear magnetic resonance. The results show that in the “crystalline” opals studied, some 90 per cent or more of the total water is physically adsorbed whereas in “amorphous” opals, at least 20 per cent but perhaps much more of the total water is held as hydroxyl groups chemically bonded to the silica surface. The rate of water loss on heating is also different, being chiefly controlled by the pore structure in “crystalline” opals but to a significant extent by the surface structure in “amorphous”.
Formation of Australian opal
Authors: Thomas, Paul & Aldridge, Laurie
Published in: InColor Magazine, 8(41), 62–69
Opal is a hydrous silica composed of predominantly silicon dioxide and water. The chemical composition of opal is normally described by the general formula SiO2.nH2O. The formula indicates that opal contains water and the value of ‘n’ is variable so the water content is variable and is known to range widely. Such a simple formula hides much of the important characteristics of how water is contained in opal and the variability in the water content and states of water is intricately involved in the formation of opal and may influence properties of the opal as a gemstone. The understanding of the states of water in opal is therefore of importance. The way in which the water is contained provides clues to the mechanisms of formation of opal. The water contained may also be used as a probe to help elucidate the complex microstructure beyond the sphere array structure in which precious opal, in particular, is described. This article will outline the types of water present in opal that displays play-of colour (POC) and how these types have been determined using chemical and physical laboratory characterisation techniques.
Authors: Dowell, K, Mavrogenes, J, McPhail, D et al
Published in: Regolith and Landscapes in Eastern Australia Conference 2002, ed. Ian C Roach, CRC LEME, Bentley, WA, pp. 18-20.
Black opal, the most unique and economically important opal in the world, is only found at Lightning Ridgein northern New South Wales. Only a handful of studies have been published on black opal, all of whichsuggest that black opal formed in the Cretaceous and Early Tertiary (Darragh et al. 1965, Watkins 1984,Pecover 1996, Behr 2001, Behr et al. 2000, Townsend 2001). Determining the origin of black opal isimportant for our understanding of sedimentation and regolith evolution, silica transport pathways and toimprove the value of the mineral resource economy of New South Wales. This study aims to determine theage and origin of Lightning Ridge black opal.
Authors: Herrmann, J. R., Maas, R., Rey, P. F., & Best, S. P.
Published in: Australian Journal of Earth Sciences, 66(7), 1027–1039
Black opal (opal-AG) owes its dark coloration to a fine-grained pigment commonly inferred to be mainly carbon, yet chemical compositions for black opals suggest there may be additional components. Here we search for such components in pigment concentrates prepared by dissolving black opal nodules (nobbies) from Lightning Ridge (NSW) in hydrofluoric acid, using electron microscopy (scanning electron microscopy, transmission electron microscopy), X-ray diffraction and laser-ablation ICP-MS. The results demonstrate the presence of sulfides—predominantly pyrite and chalcopyrite, with minor galena and Ti-oxide phases, as additional components of the pigment. ATR-FTIR analysis indicates the presence of C=O and C–H groups, consistent with an organic origin. Transmission electron microscopy images of pigment show variously deformed, originally spherical ∼100 nm particles rich in sulfide and carbon, which are interpreted as thin coatings of pigment on now dissolved opaline silica spheres. Laser-ablation ICP-MS analysis identifies remnant silica in pigment concentrates, which may be interpreted as opaline silica surviving HF treatment protected as inclusions in sulfides. When examined within the context of petrographic observations from more than 1000 opal nodules (nobbies) at Lightning Ridge, these new results suggest that pigment carbon and sulfides in the nodules formed microbially under initially anoxic groundwater conditions, within pre-existing cavities concurrently being filled with silica sol ultimately derived from chemical weathering of feldspar-rich volcaniclastic sediment. Intensely black pigment layers observed at the floor of many nodules indicate settling of dark, high-density (sulfide–Ti-oxide-rich) pigment within cavities, with the implication that sulfate-reducing bacterial (SRB) activity commences early during the silica sol-gel ripening process. Microbial activity may persist until after the cavity has completely filled with the silica sol, as illustrated by abundant black opals with uniformly distributed pigment. Pigment formed at this stage may no longer be able to settle out within the ripening and increasingly viscous silica gel, thus forming pigmentation throughout the opal cavity. The existence of ‘amber’, pigment-poor opal with intensely black basal pigment layers is interpreted as signalling a lack of sulfate to sustain further SRB activity, or a change to more oxidising conditions, possibly related to interaction with surface waters within a downward-penetrating weathering front. A change in redox conditions would shut off activity of SRB and thus sulfide pigment production and allow development of aerobic microbial activity as described by others.
Keywords: black opal, Lightning Ridge, black pigment, sulfides, sulfate reducing bacteria (SRB), biofilm, pyrite, chalcopyrite
Authors: Dickson, B. L.
Published in: Australian Journal of Earth Sciences, 66(5), 645–655.
The means and timing of the formation of Australian sediment-hosted precious opal remain a subject of continuing debate. In this study, the question of which water formed the opal is addressed by examination of rare earth element data for opals and host rocks. The available data, mainly for Lightning Ridge, NSW, suggest a positive Eu anomaly, relative to the neighbouring Sm and Dy, occurs in opals whereas no such anomaly was found for the weathered Cretaceous sediments hosting the opal. Such anomalies may be inherited from the source rock with a similar positive Eu anomaly or generated in situ by severe reduction. There is no indication of major reduction processes during the opal formation that could have led to such a Eu anomaly so this is likely inherited from a source rock. As the opal host rocks did not show this anomaly, the source rocks must be external to the opal fields. Calcite cements within rocks hosting the aquifers of the Eromanga and Surat basins of the Great Artesian Basin have been reported to have a positive Eu anomaly, which strongly suggests that opal was formed by upwelling Great Artesian Basin artesian waters. This work has also highlighted variations in trace-element concentrations in opals, which indicate significant variation in the source water composition during opal formation or different water sources were involved. Either of these is indicative of the source for the opal with its trace elements derived from external sources. These conclusions have significant implications to considerations of how opal formed, and hence, for the exploration for other deposits and to the chemistry and timing that led to the formation of opal.
Keywords: opal, Great Artesian Basin, europium, rare earth elements, Lightning Ridge, artesian water, Bulldog Shale, Cadne-Owie Formation
Authors: Dutkiewicz, A., Landgrebe, T. C. W., & Rey, P. F.
Published in: Gondwana Research, 27(2), 786–795.
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.
Keywords: Opal, Great Artesian Basin, LA ICP-MS, Cretaceous, Multivariate analysis
Authors: Keller, P.
Published in: Australian Journal of Earth Sciences, 60(3), 291–314.
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.
Keywords: Australian precious opal, Great Artesian Basin, acidic weathering, Mars analogue
Authors: Keller, P.
Published in: Gemstones and Their Origins (pp. 19–33). Springer US.
Water at or near the earth’s surface plays an important role in the formation of some gem minerals. Surface water is capable of dissolving many minerals, particularly when provided a great deal of time to do so. As a result, it carries away components in solution that remain dissolved until, under appropriate conditions, new minerals are deposited. Precious opal and other gemstones form from surface water under special conditions that may include chemical reactions, cooling of waters previously heated by nearby molten rock, and evaporation. Rainwater, for example, combines with atmospheric carbon dioxide to produce carbonic acid, a weak natural acid. If such water seeps into the earth and encounters sulfides (such as pyrite, FeS2), sulfuric acid, a much stronger acid, is produced, which dissolves minerals, transports their chemical elements, and permeates other rocks to form new minerals.
Keywords: South Wale, Solid Silica, Great Artesian Basin, Water Seep, Opal Deposit
Authors: Voiculescu-Holvad, Christian
Published in: Working Paper
The process of mineralization is well known in the paleontological world: pyritised, silicified and other forms of mineralized fossils are well documented in the scientific community, and are highly prized by collectors. But, the most rare and most valuable mineralized fossils are the opalised ones. This article provides an overview on the opalised fossils of Australia, focusing namely on the reptilian fauna preserved. Particular attention is given to the Lightning Ridge (New South Wales) biota, due to the faunal complexity, diversity and variety preserved mostly in the form of opalised fossils.
Authors: Watkins, J.J., Behr, H.J., Behr, K.
Published in: Quarterly Notes – Geological Survey of New South Wales
Opal from Lightning Ridge is amongst the most valuable and widely known in the world. Black opal, characterised by a dark body tone, is the rarest and most valuable. The opal occurs in geode-like ‘nobbies’ up to several cm in diameter and in seam-like structures in an Early Cretaceous volcaniclastic host rock. The host rock at Lightning Ridge consists of a finely laminated silty claystone that often has a high content of organic detritus. Strong bioturbation by nematodes is common, as are opalised macrofossils.
This study reports on the fossil microbe communities discovered within both the host rock and opal in cell numbers up to 107–108/cm3. The most common microbes are the aerobic bacteria actinomycetes (Nocardia, Streptomyces, Micromonospora) and myxobacteria. The fossil microbes (mostly preserved as moulds) occur in the form of mycels, mats, biofilms, globular colonies, networks, swarms and as individuals. The cell forms are mostly rod-shaped, ovoid and coccoid and generally range from 2–5 μm but may exceed 100μm. Small globular spores may contain organic residues with strong red fluorescence. All the microbes are autochthonous and are the same age as the opal.
The type of fossil microbe communities found in Lightning Ridge opal generally occur in soil or in organic muds deposited under still conditions or in a surface-fouling biomass. The microbes require a nutrient-rich (cellulose and chitin) near-surface aerobic environment with temperatures less than 35°C and near-neutral pH.
The microbes produced carbonic and organic acids that aided the biochemical weathering of clay minerals and feldspar to produce silica hydrosol. The kind of environment required by the microbes for life indicates the conditions under which opal was produced. This enables the determination of a new timetable for opal formation involving weeks to a few months and not the hundreds of thousands of years envisaged by the conventional weathering model. Opal is formed as part of the diagenetic process — forming at the same time as the sediments in which they are found.
Keywords: opal, formation, gel, bone, fossilized, replacement, Australia.
Authors: Pewkliang, Benjamath; Pring, Allan; Brugger, Joël
Published in: The Canadian Mineralogist 46(1):139-149
The composition and microstructure of opalized saurian bones (Plesiosaur) from Andamooka, South Australia, have been analyzed and compared to saurian bones that have been partially replaced by magnesian calcite from the same geological forma-tion, north of Coober Pedy, South Australia. Powder X-ray-diffraction analyses show that the opalized bones are composed of opal-AG and quartz. Major- and minor-element XRF analyses show that they are essentially pure SiO2 (88.59 to 92.69 wt%), with minor amounts of Al2O3 (2.02 to 4.41 wt%) and H2O (3.36 to 4.23 wt%). No traces of biogenic apatite remain after opal-ization. The opal is depleted in all trace elements relative to PAAS. During the formation of the opal, the coarser details of the bone microstructure have been preserved down to the level of the individual osteons (scale of around 100 mm), but the central canals and the boundary area have been enlarged and filled with chalcedony, which postdates opal formation. These chemical and microstructural features are consistent with the opalization process being a secondary replacement after partial replacement of the bone by magnesian calcite. They are also consistent with the opal forming first as a gel in the small cavities left by the osteons, and the individual opal spheres growing as they settle within the gel. Changes in the viscosity of the gel provide a ready explanation for the occurrence of color and potch banding in opals. The indication that opalization is a secondary process after calcification on the Australian opal fields is consistent with a Tertiary age for formation.
Keywords: opal, formation, gel, bone, fossilized, replacement, Australia.
Authors: Richard S. Mitchell, Susan Tufts
Published in: American Mineralogist (1973) 58 (7-8): 717–720.
Opalized fossil wood usually has a structure approaching high-tridymite. Only in rare cases does it resemble low-cristobalite, a structure often shown by other varieties of opal. Spectrochemical studies show that the silica of tridymite-like opals is chemically more pure than the silica of cristobalite-like or amorphous opals.These latter varieties usually contain more Al, Na, B, and Zr.
Authors: L Katona and C Krapf
Published in: Geological Survey of South Australia Department for Energy and Mining: Report Book 2020/00001. Department for Energy and Mining, South Australia, Adelaide
The Mintabie Precious Stones Field (MPSF) has, since the early 1920s, produced some of the finest quality opal in the world. This report book provides a review for understanding the remaining opal resources. A detailed spatial analysis shows that after approximately 40 years of mining in the MPSF, an area totalling less than 2 km2 has been intensively mined. A conservative estimate of the area of greatest prospectivity within the MPSF is 20 km2. Within this region of high prospectivity, assuming a similar deposit density to the intensively mined areas to date, the MPSF will support mining for ~400 years at the levels already experienced in its lifetime. Complimentary analysis published in 2002 reveals further prospective regions within the MPSF totalling 44 km2. The total value of the opal mined at Mintabie up to 2016 has been estimated by the South Australian Government to be $412M (unadjusted value of rough opal). An area-based analysis concludes that this is less than 10% of the total contained opal at Mintabie. The opal resource in the MPSF, including the opal already found, is therefore estimated to have an unadjusted raw opal value of over $4B.
Authors: Tao Hsu, Andrew Lucas, and Vincent Pardieu
Published in: Gems & Gemology, Winter 2015, Vol. 51, No. 4
With more than 170 years of opal mining and trading activity, Australia is synonymous with opal. Many world-renowned deposits are distributed in and along the margin of the Great Artesian Basin (GAB). Opal exports contribute roughly $85 million annually to the nation’s GDP. The industry employs thousands in the Outback communities and attracts more than 231,000 tourists per year. Opal tourism brings an estimated $324 million each year to these remote mining communities (National Opal Miners Association, n.d.).
To better understand the deposits, collect samples, and experience the opal culture, the authors traveled to Australia in June 2015 and visited four important opal fields: Lightning Ridge, Koroit, Yowah, and Quilpie (figure 1).
Authors: Dr Simon R. Pecover
Published in: Pan Gem Resources (Aust) Pty Ltd
One of the most defining geological features of all the opal producing areas in the Great Australian Basin (GAB) is the ubiquitous, pervasive and widespread brecciation of the Cretaceous sedimentary rocks hosting deposits of precious opal.
Zones of brecciation associated with faulting and fracturing in the opal fields occurs over distances ranging from millimetres to tens of metres, and affects both sandstones and claystones. The zones of brecciation may be horizontal (concordant to bedding) through to vertical (discordant to bedding), and are commonly highly irregular in shape.
While some forms of brecciation appear to be syndepositional in nature, resulting from erosional episodes that punctuated periods of deposition (forming layers comprising jumbled mixtures of rip-up clasts concordant to bedding), the formation of breccia bodies that are syntectonic and associated with faults and fractures are of most significance to understanding the formation of opal deposits in the GAB. Today, Lightning Ridge provides one of the best locations in Australia to study these fault and fracture hosted tectonic breccias and their genetic relationships to juxtaposed veins of potch and precious opal (Pecover 1996, 1999 & 2003).
Authors: Burton, Gary & Mason, A.
Published in: Quarterly Notes of the Geological Survey of New South Wales 107, 1-10.
The opal fields ol the White Cliffs area are situated within the Cretaceous (Aptian) Doncaster Member of the Great Australian Basin sequence. The Doncaster Member is comprised of interbedded sandy/silty claystone and sandstone. Opal occurs mainly as thin, horizontal and, less commonly, vertical seams within the sedimentary rocks. It also occupies cracks within erratic boulders and concretions as well as forming coatings on those bodies. Opal also forms casts after fossils and minerals, and occurs within and adjacent to faults. Silica-rich fluids, believed to have been produced as the result of kaolinisation of the Cretaceous rocks, pooled within joints and other voids and — over time — precipitated opal. The opal does not appear to be related to any particular lithological horizon. Hence vertical facies changes cannot be used as an opal exploration tool in the White Cliffs area, in contrast to the opal fields of Lightning Ridge. There is some evidence that faults have assisted in siliceous fluid migration and hence opal deposition. Some opal seams occur within and adjacent to faults, as shown by old opal workings developed parallel to faults —and many opal workings in the White Cliffs area occur on or adjacent to lineaments and lineament intersections identified on aerial photographs. The interpreted photolineaments trend north-easterly, north-westerly and northerly. Interpreted Landsat lineaments in the region also trend north-easterly, north-westerly and northerly and mainly reflect drainage and the limits of Cretaceous outcrop. Basement structures, interpreted from regional aeromagnetic and gravity images, trend mainly north-easterly and north- westerly. Although a direct correlation between the location of opal fields and interpreted basement structures is equivocal, it seems likely that basement structures have influenced structures in the Cretaceous rocks and in turn those structures have influenced opal deposition. Hence, the recognition of lineaments in the Cretaceous rocks is considered to be a useful opal exploration criterion. However, as many opal deposits are not apparently associated with lineaments there may be no indirect method of locating some opal deposits within the White Cliffs area. Further work to improve the understanding of opal formation in the White Cliffs area should include detailed structural analyses and detailed stratigraphic, lithological and geochemical studies. Stratigraphic relationships indicate that opal probably formed during a Maastrichtian to Early Eocene weathering event.
Keywords: opal, opal formation, White Cliffs, stratigraphy, geophysics, structures
Authors: Merdith, A. S., Landgrebe, T. C. W., Dutkiewicz, A., & Müller, R. D.
Published in: Australian Journal of Earth Sciences, 60(2), 217–229.
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.
Keywords: opal, Great Artesian Basin, data mining, data layering, prospectivity, mineral exploration, Cretaceous sedimentary rocks, Australian regolith.
Opal & Culture
Authors: Caucia, Franca; Ghisoli, Christian; Marinoni, Luigi; Bordoni, Valentina
Published in: Neues Jahrbuch für Mineralogie – Abhandlungen Band 190 Heft 1 (2012), p. 1 – 9
Throughout the ages, opal’s fascinating play of color phenomenon has inspired the fantasies of artists and the passion of connoisseurs. The term opal is derived from the ancient Greek and underwent several modifications over time, so that several synonyms appear in mineralogy and gemology texts. The opal is frequently mentioned in historical sources of Roman Age (especially in those of Pliny), and in those of the Renaissance, while is much less present in the works of the Middle Ages. In the Roman Age, the opal gem was very much appreciated, considered as a sacred stone and reached very high prices. This appreciation is confirmed by several anecdotes, such as those relating Octavius and Mark Anthony. At different times, original description and interpretations of the beauty of this gem were provided by authors like Pliny, Marbodius, Albertus Magnus, Camillo Leonardi, Pietro Caliari, Pio Naldi, Giovanni Antonio Scopoli and others. Throughout history this stone was seen, on and off, as a bearer of good or bad luck and different healing properties were assigned, such as that of treating eyesight problems, to make the person invisible, to make the birth easier. This led to different assessments of its commercial value. Beside its decorative uses, opal also assumed roles in short-lived fashions: for instance was associated with the month of October and was used in the so-called “sentimental jewels. The opal is frequently present in the literary works of many classical authors such as William Shakespeare, Walter Scott, Guillaume Apollinaire, Gabriele d’Annunzio. In particular it is well known as, in modern times, the fame of bad luck attributed to the opal is due to an approximate reading of the beautiful novel by Walter Scott “Anne of Geierstein In historical times opals came almost exclusively from the deposits of Cernowitz, in the actual Slovakia. Later, other deposits were discovered in various locations around the world and, currently, almost all of the opals on the market derive from Australia.
Keywords: opal, gemstone, history, legend, play of color, fashion, myths
Authors: Condello, Annette
Published in: Gardetti M., Muthu S. (eds) Sustainable Luxury, Entrepreneurship, and Innovation. Environmental Footprints and Eco-design of Products and Processes. Springer, Singapore.
In a world of diminishing resources, the opal has become a sign of mineral exclusivity for the consumer luxury market and its value as a luxury object comes from gemstone cognoscenti. According to one Australian Aboriginal legend, rainbow-hued opals are believed by some to stir emotions of loyalty and connection to the earth. Regarding the integral indigenous connection of Australia’s national gemstone, rarely has one has looked at the spaces where opal veins were once quarried in remote regions in terms of sustainable luxury. More importantly, the revival of South Australia’s opal mining industry in Coober Pedy by female Aboriginal entrepreneur Tottie Bryant in 1946; its development into a multi-million dollar industry into a modern hub in the 1970s; and the spread of the town’s construction of subterranean spaces a decade later, enticed immigrants to mine for opals. And when seeking an inexpensive and cool environment, the place enticed immigrants to live underground, providing an unusual form of sustainable luxury in Australia. In 1968, for instance, former Coober Pedy opal entrepreneur John Andrea planned for a unique international underground hotel, the luxurious Desert Cave, but it was not until 1981 when Umberto Coro realised the subterranean spaces’ potentiality and created Andrea’s dream. Another opal entrepreneur Dennis Ingram designed a golf course with ‘scrapes,’ which emerged above ground made with opal quarry dust and waste oil. In popular culture too, the town had attracted filmmakers, such as George Miller, to produce his post-apocalyptic epic Mad Max, and Wim Wenders, to document his wandering scenes not because of opal scarcity but due to the harsh desert-landscape littered with spoil heaps. Turning to adaptive reuse and indigenous culture in Coober Pedy, this chapter addresses the existing underground passages as the recyclable-integration of a former mining site. In tracking the way in which the community and its rural groundwork served as a site for an innovation in sustainable luxury, the remote underground passages has revealed an unusual Australian lifestyle. Concentrating on the underground spaces, the chapter tracks the manner in which the abandoned sites serve as poignant opal connections within Coober Pedy’s integration of remnant spaces and their adaptive reuse into museums. Opal museums of the future will become magnetic as tourist destinations and their conversion of remnant spaces also into educational facilities foresees the uniqueness of sustainable luxury through its existing empty quarries.
Keywords: Aboriginal luxury, Opal museums, Coober Pedy, Underground spaces, Dugouts
Authors: Gillies, A. D. S., Mudd, K. E., & Aughenbaugh, N. B.
Published in: The Potential of Earth-Sheltered and Underground Space (pp. 163–177). Elsevier.
Over seventy per cent of the Australian continent is classified as arid or semi-arid and most of this land area is subjected to extremes of temperature during the summer months. Mining for opal has been undertaken at a number of locations throughout this region for the last hundred years, and to overcome the adverse climatic conditions of living, the residents have tunneled underground to establish homes. Many subsurface housing designs have evolved over the years.
Coober Pedy is the largest opal producing center in Australia with a population of between 3000 and 4000. Underground space has been developed within the town generally by excavating laterally into hillsides. By this method a hotel, a Church, and hundreds of homes have been built underground.
While some excavations are still mined using manual techniques, mechanized hammers and continuous mining machines, which are normally employed mining for opal, are in widespread use. Using these, the mining of an underground dwelling can be completed in a few days. Exposed wall and roof surfaces are left smooth so that the “squared” room geometries resemble the interior design of a conventional surface house.
A number of underground houses have been completed with features comparable to luxury homes and have been sold at prices exceeding $100 000.00. Important considerations in the design of a house include
(a) the insulating properties of rock cover and temperature extremes experienced inside,
(b) rock stability and the design of pillars, openings and roof spans,
(c) the use of auxiliary ventilation to reduce heat build-up and atmospheric condensation,
(d) the provision for waste water and sewage drainage,
(e) the treatment of interior walls, and
(f) the consideration of noise insulation.
Results from a survey of underground home owners in Coober Pedy will be presented. These illustrate the different approaches taken to overcome problems which can arise from living underground in this environment. While the geological and climatic conditions in this area create an advantageous environment for this form of housing, many of the design considerations present here have application to subsurface construction in other parts of the world.
Historical Publications about Opal
Authors: Nehemiah Bartley
Published in: Book, Brisbane: Gordon and Gotch
At the suggestion of friends, I have herein collated, for publication, some rambling recollections, drawn from a diary that was first started in 1846. I hold that, neither the era of Dampier (circa 1690), nor of Cook (in 1770), nor of Macquarie (in 1820), bears so deep an interest for posterity as those fateful, stirring years, during which, thanks to her gold, Australia rose, from being a mere convicts’ wilderness, to become one of the most advanced and interesting countries in the world. And, besides this, not only is truth, at times, stranger, and more readable, than fiction, but a book, which is destitute, alike, of dialogue, plot, or hero, and in no way built upon the orthodox lines of the three-volume novel, may still—if it follows humbly in the wake of such guides as ” Robinson Crusoe,” or the ” Essays of Elia “—hope to find some readers ; so, I venture.
Authors: Brewster, David
Journal of the Franklin Institute, of the State of Pennsylvania, for the Promotion of the Mechanic Arts; Devoted to Mechanical and Physical Science, Civil Engineering, the Arts and Manufactures, and the Recording of American and Other Patent Inventions (1828-1851); Philadelphia Vol. 10, Iss. 3, (Sep 1, 1845): 195.
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