The vast, arid landscapes of Western Queensland have long concealed a glittering secret: the elusive and precious opal. Despite a brief period of booming prosperity that peaked in 1895, the region’s opal fields have lain largely neglected for decades. Persistent and widespread drought, coupled with the isolation of the mining areas, led to a cessation of broader prospecting and the abandonment of established mines. For many years, production has been minimal, relying mainly on the seasonal activities of fossickers working known deposits only when water supplies were favorable.

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The Geological Opportunity: A Call to the Modern Prospector

The geological potential of Western Queensland, however, remains profoundly high. This guide, originally compiled in 1966 by T. H. Connah, M.Sc., Senior Geologist, Geological Survey of Queensland, argues that there is no fundamental reason why a significant revival should not result from a more systematic approach. The region’s opal deposits occur in a belt some 550 miles long, extending from Kynuna to the southern border, within the remnants of shales and sandstones overlying the Cretaceous rocks of the Great Artesian Basin. It is within these weathered clay shales and sandstones—often capped by a hard lateritic layer—that opal deposits occur.

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Understanding the Target: Sandstone vs. Boulder Opal

For the modern prospector, success begins with distinguishing the two primary modes of opal occurrence: “sandstone” and “boulder.” Sandstone opal typically occurs as seams or pipes in a bed of sandstone, often separated from underlying clay by a ferruginous layer called the “casing.” It is found throughout the fields and has historically been worked to depths seldom exceeding 50 feet.

However, the key focus for a significant portion of the Western Queensland industry, particularly in areas like Toompine and Yowah, is boulder opal prospecting. Boulder opal is found as thin veins occupying cracks and cavities within hard, concretionary ironstone nodules, or “boulders,” embedded in the sandstone beds. These boulders can vary greatly in size and shape—from small, spherical “nuts” or “nodules” (as seen at Yowah) to large, irregular masses sometimes exceeding 10 feet in length. The opal is often deposited between succeeding concentric shells of the nodule or confined to irregular cracks within the boulder’s mass. This unique occurrence means the final gem retains the dark ironstone matrix attached to the precious opal, creating the distinct, durable, and highly prized stone known globally.

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Prospecting Methods and Modern Advantage

Historically, prospecting relied on sinking shallow shafts at random, adjacent to known ground, as visible evidence was scarce unless the opal-bearing seam or fragmented boulders (“floaters”) outcropped at the surface. With the long neglect of these fields, the information compiled in this guide—drawing on data from districts like Cunnamulla, Opalton, and Kynuna—remains highly relevant. While opal mining has always been a rigorous occupation, the present era of improved communications, motor transport, and increased equipment mobility offers a renewed opportunity to overcome the historical limiting factors of isolation and harsh weather. By understanding the specific characteristics of boulder opal prospecting—locating ironstone floaters, identifying concretionary bands, and systematically testing the promising ground—the prospector can be guided toward re-establishing the Queensland opal industry on a stable and profitable basis.

Despite a brief period of booming prosperity that saw production peak in 1895 at a value of £32,750, the region’s opal fields have lain largely neglected for decades. Persistent and widespread drought conditions, coupled with the isolation and inaccessibility of the mining areas, caused a cessation of broader prospecting and the gradual abandonment of established mines. For many years, production has been minimal, relying mainly on the seasonal activities of fossickers working known deposits only when water supplies are favorable.

However, the geological potential of the region remains high. There is no fundamental reason why a significant revival should not result from more systematic working of existing fields and a thorough prospecting of the large, untouched areas of similar country. While opal mining has traditionally been a decidedly rigorous occupation, the present era of improved communications, modern transport methods, and increased equipment mobility offers a renewed opportunity for the industry. These advancements should enable the industry to sustain itself more effectively against the historically limiting factor of harsh weather conditions.

Because of continued interest and persistent requests for updated information on these promising areas, this guide—originally compiled in 1966 by T. H. Connah, M.Sc., Senior Geologist, Geological Survey of Queensland—brings together decades of knowledge. It draws from published and unpublished reports by officers of the Geological Survey and Inspectors of Mines, including more recent prospecting data from the Cunnamulla, Opalton, Mayneside, Jundah, and Kynuna districts. This compiled information is not considered outdated, given the long neglect of the fields. It is offered here to aid the modern prospector in re-establishing the Queensland opal industry on a stable basis.

OPAL DEPOSITS

The opal deposits of Western Queensland occur in a belt some 250 miles in width lying to the west of the termini of the western railway system and extending from Kynuna, 80 miles west-north-west of Winton, in a south-south-easterly direction to the border at Hungerford, a distance of 550 miles. The beds in which opal is found form low ranges, tablelands and flat-topped hills up to 200 ft. in height, which are the remnants of a formerly extensive series of shales and sandstones overlying unconformably the Cretaceous rocks of the Great Artesian Basin. The uppermost beds in this series form a hard lateritic capping up to 50 ft. in thickness, overlying weathered clay shales and sandstones in which opal deposits occur. Frequently the clays ar silicified to form porcellanite.

While the various forms of common opal are found abundantly throughout, precious opal is restricted to patches of small extent. The mode of its occurrence varies considerably from place to place but, in general, two classes—”sandstone” and “boulder”—may be distinguished. In a typical deposit of sandstone opal a bed of sandstone is separated from underlying clay by a thin layer, up to 2 inches in thickness, of highly ferruginous material called the “casing”. Immediately above its base, the sandstone is hard and ferruginous for some inches and breaks away readily from the softer beds above. This is known as the “band”. Opal as “seams” and “pipes” occurs variously in the sandstone, band, casing, or clay and has been worked to depths below the surface seldom exceeding 50 ft. Sandstone opal in either form is found throughout the fields, frequently in the same locality as boulder deposits. Seam opal appears to have been formed by replacement of gypsum which is present in the beds, but pipe opal occurs usually as an infilling of cylindrical cavities, although some replacement of woody tissue has been noted.

VARIETIES OF OPAL

There are a great many varieties of precious opal found, some of the different kinds being distinguished by their “pattern” as pin-fire, flash-fire, or flash opal, when the colour shows as a single flash or in very large pattern, and harlequin when the colour is in small squares or chequers, the more regular the better. The latter is the most uncommon, and also the most beautiful.

A great variety of precious opal is present in the boulders. Much of that from the centres has a transparent or translucent body, so that the attached matrix plays an important part in the quality and brilliance of the stone. At the Hayricks Mine, Cribb noted that some stones carry numerous inclusions of “sand” or opaline impurities, while in others the body is smoky and the fire less intense. Stellate, arborescent, banded and other fanciful forms were found in the vertical veins. Smaller stones showing the much-prized red fire were obtained, mostly from short radial and concentric cracks near the outer margin. A very large proportion of all of the opal obtained, however, is valueless, being common opal or of the glassy bluish variety containing little or no colour.

YOWAH AND CUNNAMULLA DISTRICTS

The Yowah Opal Field is in the Cunnamulla Mining District, on the northern side of Sheep Station Creek, 10 miles from its junction with Yowah Creek of which it is a tributary. The rock consists of the pink sandstone of the lower strata of the Desert Sandstone series, and this, in many places, is covered by the characteristic debris of hard round boulders. The discovery of opal at the Great Extended was made by Mr. W. Evans when deepening an old shaft that had been left by some former prospector, and it was one of the richest deposits worked in Queensland. The main shaft on the lease was about 26 feet deep, and passed through a soft pink sandstone. At about 25 feet the rock became rather harder and had a yellow colour, and the kernel band was met with. This consists of a bed of concretionary nodules closely packed in soft clayey sandstone. The compacted mass forming this bed of kernel boulders varies from 6 inches up to 2 feet in thickness, and has a conglomerate-like appearance.

[… Verbatim continuation of the 1966 Register following the Master Format logic …]