Australian boulder opal is most celebrated for its spectacle of rainbow-like hues, which vary with lighting or angle of observation. Not every opal has this rare color feature.
The name opal evolved from the Roman word "opalus" which traced its roots from the Greek's "opallios," meaning to see a change of color. This Greek word is likewise a revision of the ancient Indian Sanskrit's "upala," which means precious stone. The Australian Boulder opal is particularly stunning because it exhibits a dark body tone which adds vibrancy in its show of color. Based on scientific estimates, it takes five million years to produce an opal just one centimeter thick. This is a solid opal and can be found in cracks and cavities of ironstone. The opal occurs either as a strong color on top of the ironstone or as flecks of hues dispersed within the entire stone. With high density due to ironstone content, Boulder Opal is more resilient than other opals. This gemstone is a gel form of silica with varying percentages of water.
Australian boulder opal is a single-source gemstone and only appears in deposits among the Cretaceous rocks in the western part of Queensland in Australia. More than 60 million years ago, an inland sea covered parts of Australia with stone sediment deposits. By the mid-tertiary period, water masses that included silica flooded back resulting in cavities and niches in the sedimentary rocks. Over time, the silica stone became Opal.
Boulder opals were first sighted in 1849 in Tarravilla, a cattle station in Australia. However, prospectors started coming in 1890. Australia produces almost 95 percent of the world's opals, of which only two percent is boulder opal.