Historical Reading List: Australian Opal

Dr. James Shigley

January 1, 2024

Opal appears to have been first discovered in 1849 in Australia near the small town of Angaston in the territory of South Australia. Within a few decades, major deposits were found and exploited in several areas of the country. Initially rough opal was sent to Germany to be cut and polished; later it was manufactured locally. Australian opal began to appear in the international market in the 1890s, and over time the availability of high-quality material made this country the world’s main producer with more than 90% of the supply. The principal opal deposit are Lightning Ridge and White Cliffs in New South Wales; Eromanga, Opalton, Quilpie, Winton and Yowah in Queensland; and Andamooka, Coober Pedy, Lambina and Mintabie in South Australia.

How to Use this Reading List

This reading list gives you an opportunity to learn more about the history of opal from Australia. Entries in the list are presented in chronological order to emphasize the development of ideas over time. The list is not comprehensive, but is a compilation of some interesting information that has often been forgotten or overlooked.

Many of the articles in the reading list exist in the public domain and can be found online at digital libraries such as Hathitrust, Internet Archive, or other digital repositories.  More recent publications can often be found in libraries.  Abstracts of these more recent articles can usually be found on the website of the original journal or magazine, and the article itself is often available for purchase from the publisher.
 
The Occurrence of Opals in Central Australia and Queensland, J.R.M. Robertson, Chemical News, Vol. 45, No. 1162, pp. 95-97 and No. 1163, pp. 101-104, (1882). The author describes a visit to the areas where opal is found and its mode of occurrence.

The Opal King and His Country, Author unknown, Pall Mall Budget, Vol. 35, No. 986, p. 17, (1887). A summary of an interview with H.W. Bond, who in about 1880 was one of the first individuals to mine opals in Queensland near Cooper’s Creek, and who was visiting London at the time to raise funds for the mining operations.

Some Notes on the White Cliffs Opal Fields, Wilcannia, and The White Cliffs Opal Fields, New South Wales, F.G. de V. Gipps, Transactions of the Australasian Institute of Mining Engineers, Vol. 2, pp. 70-80, (1894), and Engineering and Mining Journal, Vol. 59, No. 19, pp. 437-438, (1895). An early description of the opal fields, which had been found just a few years earlier in 1890. The author discusses the occurrence of opal and the type of material being found. A short summary of this account appeared in the Engineering Magazine, Vol. 9, No. 5, p. 975, (1895).

On the Occurrence of Precious Stones in New South Wales, and the Deposits in Which They are Found, J.M. Curran, Journal and Proceedings of the Royal Society of New South Wales, Vol. 30, pp. 214-285, (1897). The author describes the deposits of opal and other gems in the colony of New South Wales.

The Mineral Resources of New South Wales, E.F. Pittman, Geological Survey of New South Wales, pp. 398-405, (1901). This review article contains a section on the opal deposits of the territory.

Edelopal und Opal-Pseudomorphosen von White Cliffs, Australien [Gem Opal and Opal Pseudomorphs from White Cliffs, Australia], G. Gürich, Neues Jahrbuch für Mineralogie, Geologie und Palaeontologie, Vol. 14, pp. 472-483, (1901). The author describes some of the unusual opalized pseudomorphs of other minerals found in the White Cliffs area.

Australian Opal and Opal Mining, J. Plummer, Scientific American Supplement, Vol. 52, No. 1342, pp. 21505-21506, (1901). A brief description is given of the opal mining areas. The same article appeared in the Journal of the Society of Arts, Vol. 49, No. 2541, p. 685, (1901); a version also appeared in the Mining Reporter, Vol. 44, No. 3, p. 39, (1901).

Opal-Fields of New South Wales, Author unknown, Chambers’s Journal, Vol. 79, No. 240, pp. 493-496, (1902). The author describes a visit to White Cliffs and of the variety of opal material found there.

The Opal Mining Industry, and the Distribution of Opal Deposits in Queensland, C.F.V. Jackson, Queensland Department of Mines, Geological Survey Report, No. 177, 34 pp., (1902). This report presents information on the occurrences and mining of opals in the territory.

On the Trail of the Opal, A. MacDonald, Pall Mall Magazine, Vol. 32, No. 129, pp. 40-49, (1904). A report of an expedition conducted in 1903 to the interior regions of Australia to look for opal.

A Chapter on Opals, H.K. Walker, The Nineteenth Century and After, Vol. 56, pp. 492-498, (1904). General information is given on opal including a history of its discovery in Australia.

An Opalized Dogfish, Author unknown, Chambers’s Journal, Vol. 81, No. 347, pp. 542-543, (1904). Since their discovery, the White Cliffs opal fields have yielded opalized marine fossils including, as reported here, a fossilized member of the shark family.

The Opal Formations of Australia, R.M. MacDonald, Scottish Geographical Journal, Vol. 20, No. 5, pp. 253-261, (1904). A geographical description is given of the opal mining regions in Queensland and South Australia.

Opal Pseudomorphs from White Cliffs, New South Wales, C. Anderson and H.S. Jevons, Records of the Australian Museum, Vol. 6, No. 1, pp. 31-37, (1905).  Based upon a study of the so-called “fossil pineapples” found at White Cliffs, the authors concluded that they represent pseudomorph clusters of glauberite crystals that had been replaced by opal.

The White Cliffs Opal Field, New South Wales, J. Plummer, Mining and Engineering World, Vol. 29, No. 12, pp. 445-446, (1908). The author discussed the geological setting of the opal deposits.

The Black Opals of Australia, I. Ramsay, Mid-Pacific Magazine, Vol. 11, No. 2, pp. 122-125, (1916). Black opals were found at Lightning Ridge in about 1905. A description is given of opal mining in this part of New South Wales.

Some New Facts about the Queensland Opal Fields, Author unknown, Jewelers’ Circular, Vol. 77, No. 4, p. 67, (1918). A summary is given of opals mining operations.

Black Opal Mining at Lightning Ridge, Author unknown, Jewelers’ Circular, Vol. 80, No. 13, p. 75, (1920). A brief description is given of the method of opal mining.

“Opal: The Gem of the Never Never”, T.C. Wollaston, T. Murby and Company, London, (1924). Book not seen; a review appeared in Nature, Vol. 115, No. 2887, pp. 292-293, (1925).

“Opals and Gold: Wanderings and Work on the Mining and Gem Fields”, R.M. Macdonald, J.B. Lippincott Company, Philadelphia, 256 pp., (1928). The author describes a visit to the opal and gold mining regions.

The Occurrence and Formation of Opal at Coober Pedy and Andamooka,  J.B. Jones and E.R. Segnit, Australian Journal of Science, Vol. 29, No. 5, pp. 129-133, (1966). The mode of occurrence of opal in these two mining regions is described.

Origin of Precious Opal, P.J. Darragh, A.J. Gaskin, B.C. Terrell and J.V. Sanders, Nature, Vol. 209, No. 5018, pp. 13-16, (1966). The authors discuss the deposition of silica in sedimentary rock formations as a basis for opal formation.

The Story of White Cliffs Opal Field, J.S. Taylor, Gems & Gemology, Vol. 14, No. 11, pp. 334-343, (1971). Based on visits to the area in 1960 and 1969, the author described the history of the mining area and the methods used to recover opal.

Andamooka Opal Fields, E.A. Hope, Australian Lapidary Magazine, Vol. 9, No. 4, pp. 29-33, (1972). A brief description of the Andamooka field is given.

Classification and Nomenclature of Precious Opal, N.A. Clayton, Australian Gemmologist, Vol. 12, No. 5, pp. 152-154, (1975). A system to categorize the various types of precious opal is presented.

Geology and Magnetic Characteristics of Precious Opal Deposits, Southwest Queensland, B.R. Senior, D.H. McColl, B.E. Long, and R.J. Whiteley, BMR Journal of Australian Geology and Geophysics, Vol. 2, pp. 241-251, (1977).  Article not seen.

“A Field Guide to Australian Opals”, B. O’Leary, Gemcraft Publications, Victoria, Australia, (1977). Book not seen.

Economic Geology of Australian Gemstone Deposits, P.L. Broughton, Minerals Science and Engineering, Vol. 11, No. 1, pp. 3-21, (1979). This article reviews the geologic setting of Australian gem deposits including opal.

Les Opales Noires de Lightning Ridge, Australie [The Black Opals of Lightning Ridge, Australia], C. d’Hotel, Monde et Mineraux, No. 52, pp. 39-41 and No. 53, pp. 32-35, (1983). This two-part article gives a description of the mining area.

L’Opale d’Australie [The Opals of Australia], Y. Dabek, Monde et Mineraux, No. 65, pp. 32-36, (1985). This short review article discusses the Australian opals and their deposits.

L’Opale Précieuse en Territoire de Sud-Australien: Coober Pedy [Precious Opal of the South Australia Territory: Coober Pedy], P. Fumey, Revue de Gemmologie a.f.g., No. 86, pp. 17-20, (1986). The author discusses the geological occurrence of opal in Coober Pedy and relates it to other Australian opal deposits.

Lightning Ridge – The Nursery of the Australian Black Opal, E. Gübelin, Swiss Journal for Watchmakers and Jewelers, Vol. 8, pp. 18-33, (1987). This article discusses the history, geologic setting, and mining methods used to recover black opal. Color photographs of both the area and opal samples are included.

The Galaxy – Australia’s Greatest Opal? M. Campbell, Australian Mineralogist, Vol. 5, (January-March), pp. 25-30, (1990). An estimated 550-carat gem opal found in 1989 near Jundah, Queensland, is described.

The Mintabie Opalfield, I.J. Townsend, Australian Gemmologist, Vol. 18, No. 1, pp. 7-12, (1992). A description is given of the opal deposits around Mintabie.

Queensland Boulder Opal, R.W. Wise, Gems & Gemology, Vol. 29, No. 1, pp. 4-15, (1993). This article gives a history, geological description of the occurrences, and the mining of boulder opal.

A New Era for Opal Nomenclature, A. Smallwood, Australian Gemmologist, Vol. 19, No. 12, pp. 486-496, (1997). A nomenclature classification developed in cooperation of gemologists and members of the Australian opal industry.

Characterization of Sedimentary Opals by Fourier Transform Raman Spectroscopy, A.G. Smallwood, P.S. Thomas and A.S. Ray, Spectrochimica Acta A, Vol. 53, No. 13, pp. 2341-2345, (1997). This article compares the infrared spectra of opals from various sources in Australia with those from other countries.

Trace Elements in Precious and Common Opals using Neutron Activation and Analysis, G.D. McOrist and A. Smallwood, Journal of Radioanalytical and Nuclear Chemistry, Vol. 223, No. 1/2, pp. 9-15, (1997). A study of trace elements in precious and common opals from several Australian deposits.

Weathered-Profile-Hosted Precious Opal Deposits, B.R. Senior, AGSO Journal of Australian Geology and Geophysics, Vol. 17, No. 4, pp. 225-227, (1998). The author lists the geological characteristics of the sedimentary opal deposits.

Fella Down a Hole, R. Hughes, Time Magazine (South Pacific Edition), Vol. 154, No. 1, pp. 64-65, (1999). This is a brief article about opal mining at Coober Pedy.

A Brief Look at Australia’s Opal Mining History, Unknown author, Australian Gold, Gem & Treasure, Vol. 15, No. 6, pp. 49-51, (2000). A chronology of the development of opal mining.

A Preliminary Investigation of Precious Opal by Laser Raman Spectroscopy, A. Smallwood, Australian Gemmologist, Vol. 20, No. 9, pp. 363-366, (2000). A Raman spectroscopy characterization of precious opal.

The Geology of Australian Opal Deposits, I.J. Townsend, Australian Gemmologist, Vol. 21, No. 1, pp. 34-37, (2001). A brief discussion is presented on the geological setting of the opal occurrences.

Opalised Sea Monsters from South Australia, B. Kear, Metal Stone & Glass Magazine, Vol. 15, pp. 20-23, (2001). This article describes some opalized fossil skeletons of Cretaceous-age creatures that have been discovered in Australian opal deposits.

Some Lesser Known Australian Opals, G. Brown, Zeitschrift der Deutschen Gemmologischen Gesellschaft, Vol. 51, No. 2/3, pp. 97-106, (2002). Some unusual kinds of opal are described.

Australia’s Gemstone Resources and Their Markets, and Update on Australia’s Gemstone and Pearl Resources, G. Brown, Australian Gemmologist, Vol. 20, No. 12, pp. 534-539, (2000) and Vol. 21, No. 8, pp. 273-277, (2002). Status reports are presented of the mining of gems in Australia including opal.

Thermal Characteristics of Australian Sedimentary Opals, L.D. Brown, A.S. Ray, P.S. Thomas and J.P. Guerbois, Journal of Thermal Analysis and Calorimetry, Vol. 68, No. 1, pp. 31-36, (2002). The authors studied the water dehydration behavior in heated Australian opals.

Australian Sedimentary Opal – Why is Australia Unique? D. Horton, Australian Gemmologist, Vol. 21, No. 8, pp. 278-294, (2002). The sedimentary opal deposits found in the central part of the country occur in generally flat-lying horizontal layers within 30 meters of the land surface. The author discusses the unique geologic conditions that produced these extensive deposits.

Capabilities of Laser Ablation Mass Spectrometry in the Differentiation of Natural and Artificial Opal Gemstones, E. Erel, F. Aubriet, G. Finqueneisel and J.F. Muller, Analytical Chemistry, Vol. 75, No. 23, pp. 6422-6429, (2003). The authors demonstrate the use of minor and trace element chemistry to distinguish between natural and synthetic opals.

The Origin of Precious Opal: A New Model, B. Deveson, Australian Gemmologist, Vol. 22, No. 2, pp. 50-58, (2004). This model envisions silica spheres that form in solution become concentrated and purified by filtration by clays that line void spaces in sedimentary rocks where the precious opal can then form.

Elemental Analysis of Australian Amorphous Banded Opals by Laser-Ablation ICP-MS, L.D. Brown, A.S. Ray and P.S. Thomas, Neues Jahrbuch für Mineralogie Monatshefte, No. 9, pp. 411-424, (2004). A trace-element chemical analysis of various color bands in Australian opals.

Old Opal Fields Revisited at Coober Pedy, I.J. Townsend, Australian Gemmologist, Vol. 22, No. 11, pp. 475-478, (2006). A report of a renewed effort using a core drilling program to locate additional opal occurrences in the vicinity of Coober Pedy.

An Archaic Crested Plesiosaur in Opal from the Lower Cretaceous High-Latitude Deposits of Australia, B.P. Kear, N.I. Schroeder and M.S.Y. Lee, Biology Letters, Vol. 2, No. 4, pp. 615-619, (2006). Description of the discovery of an opalized fossil.

Interpretation of Palaeoweathering Features and Successive Silicifications in the Tertiary Regolith of Inland Australia, M. Thiry, A.R. Milnes, V. Rayot and R. Simon-CoinÇon, Journal of the Geological Society, Vol. 163, No. 4, pp. 723-736, (2006). The authors present evidence of the geological conditions in sedimentary rocks that result in the formation of opal.

A SIMS Study of the Transition Elemental Distribution between Bands in Banded Australian Sedimentary Opal from the Lightning Ridge Locality, P.S. Thomas, L.D. Brown, A.S. Ray and K.E. Price, Neues Jahrbuch für Mineralogie Abhandlungen, Vol. 182, No. 2, pp. 193-199, (2006). Data are presented on the distribution of trace elements in banded sedimentary opal.

Australian Opal Resources: Outback Spectral Fire, S.R. Pecover, Rocks & Minerals, Vol. 82, No. 2, pp. 103-115, (2007). The author discusses in some detail the occurrence, geologic theories of origin, and methods of mining opal.

Natural Gamma Radioactivity and Exploration for Precious Opal in Australia, B.R. Senior and L.T. Chadderton, Australian Gemmologist, Vol. 23, No. 4, pp. 160-176, (2007). Radioactivity measurements in drill cores and mine walls can be used to detect opal mineralization in Australia.

TMA and SEM Characterization of the Thermal Dehydration of Australian Sedimentary Opal, A. Smallwood, P.S. Thomas, A.S. Ray and P. Šimon, Journal of Thermal Analysis and Calorimetry, Vol. 88, No. 1, pp. 185-188, (2007). A characterization study of the behavior of white opal during controlled heating up to 1000°C.

Estimation of the Diffusion Coefficient of Water Evolved during the Non-Isothermal Dehydration of Australian Sedimentary Opal, P.S. Thomas, P. Šimon, A. Smallwood and A.S. Ray, Journal of Thermal Analysis and Calorimetry, Vol. 88, No. 1, pp. 231-235, (2007). The loss of water of powder and bulk material during heating of an opal specimen was investigated by thermogravimetric analysis.

Nanoindentation Hardness of Banded Australian Sedimentary Opal, P.S. Thomas, A.S. Smallwood, A.S. Ray, B.J. Briscoe and D. Parsonage, Journal of Physics D: Applied Physics, Vol. 41, No. 7, Article 074028, (2008). Despite differences in appearance and microstructures, various opal samples exhibited little difference in mechanical properties such as indentation hardness.

The Formation of Precious Opal: Clues from the Opalization of Bone, B. Pewkliang, A. Pring and J. Brugger, Canadian Mineralogist, Vol. 46, No. 1, pp. 139-149, (2008). The authors compare the microstructure of fossil bones which were and were not opalized after sediment deposition, and their observations suggest that opalization is a secondary process after bone calcification in the Australian opal fields consistent with a Tertiary age for fossil formation.

Thermal characterisation of the Dehydration of Australian Sedimentary and Volcanic Precious Opal, A.G. Smallwood, P.S. Thomas and A.S. Ray, Journal of Thermal Analysis and Calorimetry, Vol. 92, No. 1, pp. 91-95, (2008). The authors compare the behavior of opals from two geologically distinct environments, and they found significant differences in dehydration rates.

Comparative Analysis of Sedimentary and Volcanic Precious Opals from Australia, A.G. Smallwood, P.S. Thomas and A.S. Ray, Journal of the Australian Ceramic Society, Vol. 44, No. 2, pp. 17-22, (2008). A comparison of sedimentary and volcanic opals using several characterization techniques.

Opal Similarities between Andamooka and Coober Pedy, South Australia, I.J. Townsend, Australian Gemmologist, Vol. 23, No. 9, pp. 415-420, (2009). A comparison of opal samples from two important localities.

Opalized Archosaur Remains from the Bulldog Shale (Aptian: Lower Cretaceous) of South Australia, P.M. Barrett, B.P. Kear, R.B.J. Benson, Alcheringa: An Australasian Journal of Palaeontology, Vol. 34, No. 3, pp. 293-301, (2010). Description of an opalized fossil reptile.

Thermal Properties of Australian Sedimentary Opals and Czech Moldavites, P.S. Thomas, J. Šesták, K. Heide, E. Fueglein and P. Šimon, Journal of Thermal Analysis and Calorimetry, Vol. 99, No. 3, pp. 861-867, (2010). Comparison of the thermal properties of sedimentary opals and impact silica glass (moldavites).

Opalisation of the Great Artesian Basin (Central Australia): An Australian Story with a Martian Twist, P.F. Rey, Australian Journal of Earth Sciences, Vol. 60, No. 3, pp. 291-314, (2013). A discussion is presented of the unique conditions of sedimentary opal formation in central Australia, and how very similar conditions may exist on the surface of Mars.

Peculiarities of the oxygen isotope ratio in precious opals, S.V. Vysotsky, A.V. Ignatiev, A.G. Khlestunova, T.A. Velivetskaya and A.S. Okrugin, Russian Journal of Pacific Geology, Vol. 7, No. 6, pp. 427-430, (2013). In a study of opals from Australia, Ethiopia and Russia, the authors suggest that the oxygen isotope ratio in opals may serve as a criterion for estimating the sedimentary or volcanic formation temperatures. They found that the higher the formation temperature, the lighter the oxygen isotope ratio of the precipitating opal.

Relationships between Palaeogeography and Opal Occurrence in Australia: A new Data-Mining Relationship, T.C.W. Landgrebe, A. Merdith, A. Dutkiewicz and R.D. Müller, Computers and Geosciences, Vol. 56, pp. 76-82, (2013). The authors present a novel methodology, based on the open-source software in which age-coded digital palaeogeographic maps are used to “data-mine” spatio-temporal patterns related to the occurrence of Australian opal. Our aim is to test the concept that only a particular sequence of depositional/erosional environments may lead to conditions suitable for the formation of gem-quality sedimentary opal. 

Experiences on the South Australian Mintabie Opal Fields - Past and Present, P. Blythe, M. Novelli and T. Coldham, Australian Gemmologist, Vol. 25, No. 2, pp. 46-54, (2013). A discussion of opal mining around the town of Mintabie.

Towards a Predictive Model for Opal Exploration using a Spatio-Temporal Data Mining Approach, A.S. Merdith, T.C.W. Landgrebe, A. Dutkiewicz, R.D. Müller, Australian Journal of Earth Sciences, Vol. 60, No. 2, pp. 217-229, (2013). Currently there is no formal exploration model in Australia for opal, and its formation in the geological environment is poorly understood. Here the authors 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. The methodology presented uses all known locations where opal has been mined to date, which when combined with various geological information, has allowed the authors to create the first opal prospectivity map for the region.

Australian Sedimentary Opal-A and its Associated Minerals: Implications for Natural Silica Sphere Formation, M. Liesegang and R. Milke, American Mineralogist, Vol. 99, No. 7, pp. 1488-1499, (2014). Precious opals owe their ‘play-of-color’ appearance to the diffraction of light by closed-packed arrays of tiny silica spheres. The authors studied 20 opal samples from both Andamooka and Yowah using several techniques, and they concluded that the local host rocks and associated minerals are the key to unraveling the complex history of sedimentary opal formation.

The Unique Attributes of Australian Precious Opal, A. Smallwood, Australian Gemmologist, Vol. 25, No. 6/7, pp. 207-230, (2014). The author discusses the gemological features of precious opal. A similar article by the authors appeared in InColor Magazine, No. 25, pp. 36-45, (2014).

Splendor in the Outback: A Visit to Australia’s Opal Fields, T. Hsu, A. Lucas, V. Pardieu, Gems & Gemology, Vol. 51, No. 4, pp. 418-427, (2015). The authors describe a visit to the opal mining areas.

Origin of Silica and Fingerprinting of Australian Sedimentary Opals, A. Dutkiewicz, T.C.W. Landgrebe and P.F. Rey, Gondwana Research, Vol. 27, No. 2, pp. 786-795, (2015). For most of the opals studied, the authors suggest that silica is most likely derived locally from the opal host rocks, which impart a unique elemental signature on the opal at any locality. They discuss how the combination of analytical and statistical methods provides a powerful tool for a wide range of provenance studies where relationships between major and trace elements are difficult to unravel.

Australian Opal Centre: A Glittering Palace for the Queen of Gems, J. Brammall, Australian Gemmologist, Vol. 25, No. 9, pp. 304-3156, (2015). A new tourism attraction and educational center in Lightning Ridge provide knowledge and information on Australian opal.

Raman, FT-IR and XRD Investigation of Natural Opals, A. Sodo, A.  Casanova-Municchia, S. Barucca, F. Bellatreccia, G. Della-Ventura, E. Butini and M.A. Ricci, Journal of Raman Spectroscopy, Vol. 47, No. 12, pp. 1444-1451, (2016). The authors investigated nine gem opals from the main world deposits (Australia, Madagascar, Slovakia, Mexico, Honduras and Ethiopia) using a variety of characterization techniques, and they discuss the use of their results within the context of a country-of-origin determination.

Which Water formed Australian Sediment-hosted Precious and Potch Opal? B.L. Dickson, Australian Journal of Earth Sciences, Vol. 66, No. 5, pp. 645-655, (2019). The authors found variations in trace-element concentrations in precious and common opals, and suggest these results indicate either significant variations existed in the source water composition during opal formation, or that different water sources were involved.

The Nature and Origin of Pigments in Black Opal from Lightning Ridge, New South Wales, Australia, J.R. Hermann, R. Maas, P.F. Rey and S.P. Best, Australian Journal of Earth Sciences, Vol. 66, No. 7, pp. 1027-1039, (2019). After examining within the context of petrographic observations more than 1000 opal nodules from Lightning Ridge, the authors suggest that carbon and sulfide mineral black pigments. These pigments were produced by microbial action under initially anoxic groundwater conditions within pre-existing cavities concurrently being filled with silica material ultimately derived from chemical weathering of feldspar-rich volcaniclastic sediment.

Formation of Sediment-Hosted Opal-A_G at Lightning Ridge (New South Wales, Australia): Refining the Deep Weathering Model, J. Herrmann and R. Maas, Journal of Geology, Vol.130, No. 2, pp. 77-110, (2022). The authors discuss the chemical and physical weathering conditions that resulted in the formation of opal in sedimentary host rocks.