Boulder opal, in general, is found in Queensland, Australia. Two principle opal districts take the names of the towns defining them: Quilpie and Winton. Within the Quilpie district, several major opal fields exist, including the Yowah and Koroit fields, within which most of the opal commonly known as boulder opal is to be found. But in these fields, opal is found only sporadicly, in small “patches” unsuitable for commercial exploitation. The dozen or so “working” mines in Koroit are maintained by hard working individuals endeavoring to dig the treasure out of the ground. 75% of the stones I use come from a Koroit mine owned and operated by Mr. Eugene McDevitt. He owns a classic “shaft” mine where the opal layer is found 35 to 37 feet deep. It is hard rock mining in a territory providing neither running water nor electricity. The following paragraphs are quoted from Mr. McDevitt's web site with his permission.
“Chemically, opal is hydrated silicon dioxide (SiO2·nH2O). Silicon dioxide is the most abundant compound in the Earth's crust. Common sand, quartz, flint, jasper and glass are all forms of silicon dioxide. What makes opal different is its molecular structure. Precious opal is composed of a specifically ordered arrangement of very small silica spheres with water molecules in the spaces between the spheres. This specific arrangement is a particular 3-dimensional lattice of these spheres, known as a face-centered cubic array. Different colors of opal are the result of different sized spheres. The color violet comes from an arrangement of spheres which have diameters of approximately 150 nm. The color red comes from an arrangement of spheres which have diameters of approximately 350 nm. The color of opal is the result of reflection, refraction and diffraction (known as 1,1,1– Bragg diffraction). Opal is the only gemstone whose color is not the result of impurities.
“Boulder opal appears to be sedimentary. It is commonly believed that it is formed as a result of a silica solution seeping into spaces in an iron rich type of sandstone (known as ironstone) and then solidifying and opalizing. It is not known if the silica solution simply fills in empty spaces or dissolves another mineral that was originally in those spaces. This ironstone layer or level is often located at the junction of a sandstone layer or level above and a clay layer or level below. There is some evidence that a micro-organism might play a role in the opalization process. Details of the opalization process are not well understood. There is debate about the age of opal and the length of time required for opalization. However, it is generally thought that boulder opal is one million to 10 million years old.
“The spaces in the ironstone that the opal fills in could be relatively large and flat, which could result in flat, clean-faced opals; or the spaces could be very small and scattered in a pattern throughout a nodule, which could result in Koroit or Yowah-nut type opals.”
Koroit opal is characterized by brilliant opal colors and the combination of the precious opal with other elements in the stone including colored opal potch (non-opalescent--or common opal) several densities of the surrounding matrix and patterns formed by the mix of all the different elements. The opal occurs in seams and cracks in the host ironstone as well as concretions, “nuts” and in the “opalization” of various fossils such as wood (in Koroit) shell and prehistoric bone (other Australian opal fields). Precious opal, then, is but one aspect of these stones. The variety of the look of this material is seemingly endless.
Boulder opal is rarely seen in shopping mall jewelry stores because,by its nature, it can't be easily cut into calibrated sizes to accommodate the jewelry trade. The Trade cannot tolerate unique pieces because it needs 10,000 pieces of the same size and material. Most good boulder opal goes to the orient and to Europe.
For a more in-depth discussion of boulder opal and mining. the viewer is referred to Mr. McDevitt's web site, http://www.koroit.com.
