In this week’s mineral treasure article, we take a trip to Quilpie in Queensland, Australia to discuss the beautiful gemstone opal. Quilpie is famous for its boulder opals like the specimen shown above (ﬁg. 1). This article will take a look at the unique settings in which opal can form as well as some of its special uses apart from being a precious gemstone.
A large amount of opal deposits can be found in the Great Artesian Basin of Australia (GAB). Quilpie, which is the area of interest in this article, is located near the center of the GAB in Queensland. Queensland contains over 300,000 kilometers of land, much of which is made up of opal ﬁelds. These ﬁelds produce mostly boulder opal, which is unique to this area (Hsu, Lucas, and Pardieu, 2015). In fact, Quilpie is the world’s biggest producer of boulder opal (“Queensland Opal Mining Fields”, n.d.). Opal was ﬁrst found in Queensland during the 1870s and quickly became a fashionable gemstone in Europe (Hsu, Lucas, and Pardieu, 2015). The town of Quilpie was established in April of 1917 and remains one of the main social and commercial centers in Queensland (“Queensland Opal Mining Fields”, n.d.).
The opal mineral can form from both volcanic and sedimentary deposits. Although the processes by which opals form is highly undecided upon, it is generally believed that they are produced by silicic rocks that have undergone weathering. These rocks then experience precipitation in cavities by liquids that contain large amounts of SiO2. Opals also contain water which stops them from being purely silicate minerals (Gaillou et al., 2008).
The Great Artesian Basin’s lithology mainly consists of Cretaceous age sandstone and claystone. The opals of the GAB are mainly found in these rock units. The ﬂuctuation of the sea levels during the Cretaceous period caused a withdrawal from the Australian shoreline. This played a large role in the formation of the basin due to sedimentation and subsidence. The GAB sat underneath the shallow Eromanga Sea and experienced a drastically diﬀerent climate from today during this time period. One recent theory on how opals in the GAB formed is the redox theory. This theory suggests that the Eromanga Sea was extremely cold and contained little to no carbonates. However, iron-rich and organic sediments were abundant. These sediments thrived in an environment with low oxygen and allowed for the presence of bacteria that produced pyrite. As a result of this environment, the sediments reacted strongly to oxidation. The Late Cretaceous period brought with it long periods of oxidative and acidic weathering. Redox theory proposes that opals were formed during this period of extensive weathering and were then covered and preserved by later Cenozoic sediments (Hsu, Lucas, and Pardieu, 2015).
Quilpie and the opal ﬁelds located there are a part of the heavily weathered Winton Formation. Boulder opal, which is unique to and extremely abundant in this area, is the most abundant type of opal that can be found here. It occurs in cracks in sandstone and ironstone and is usually connected to a host rock (“Queensland Opal Mining Fields”, n.d.).
Opal has a chemical formula of SiO2.nH2O and is considered to be a mineraloid because unlike true minerals, it is amorphous, meaning that it does not contain a crystal structure (Gaillou et al., 2008). Opals are a mixture of silica and water and their colors are a result of impurities within the stone. There are many diﬀerent varieties of opal. One of the rarer types of opal is the “precious” or “play-of-color” opal (“Opal”, n.d.). These are extremely valuable because of their brilliant display of rainbow colors. Their appearance is a result of a ﬂawless network of silica spheres diﬀracting light
(Gaillou et al., 2008). Another type is the ﬁre opal. These opals are usually orange, red, or yellow in their body color. Finally, there is the common opal. Common opals do not show a play-of-colors and are normally opaque. As a general rule for all types of opals, they are vitreous, resinous, or waxy in their luster. They are also normally cloudy because they are almost never transparent. Pure opals are completely colorless. Other varieties can come in brown, yellow, red, green, black, and white. Opals have a hardness of 5 to 6.5, making them one of the softer gemstones. They have a speciﬁc gravity of 1.98 to 2.25. Furthermore, they are extremely sensitive and can fracture easily when heated (“Opal”, n.d.).
Mineral Uses as a Treasure
Opals have long been considered to be a precious gemstone and are mostly used for jewelry. It was believed by the ancient Greek and Roman cultures that opals were a gift of protection from the gods and were seen to be a symbol of hope and purity (“Opal”, n.d.). Though not as common, opal can be used as a ﬁne powder abrasive for metals and as an ingredient for ﬁltration systems. They can also be found as ingredients in medicines, pharmaceuticals, cosmetics, fertilizers, and insulation
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