Opal is a globally distributed mineral, though Australia’s contribution is unrivaled in terms of volume and value. Chemically, opal is a hydrated amorphous form of silica (SiO₂·nH₂O), meaning it lacks a crystalline structure and contains water. While the vast majority of the world’s highest quality precious opal originates from Australian soil, significant deposits are mined across several other continents.
The Conditions Required for Opal Formation
Opal formation requires a precise combination of chemical elements and geological circumstances. Silica, the fundamental ingredient, is sourced from the weathering of silica-rich rocks like sandstone or volcanic ash. Groundwater dissolves this silica, creating a solution that percolates through the earth and deposits into voids or fissures within the host rock.
The slow evaporation of water over extensive geological time causes the amorphous silica gel to solidify. Precious opal exhibits “play-of-color,” which occurs when microscopic silica spheres settle into a highly ordered, three-dimensional structure. This regular arrangement diffracts light, creating the mesmerizing spectrum of colors.
Australia’s Unmatched Global Position
Australia’s dominance in the global opal market is the primary reason the assumption of exclusivity exists. The country consistently supplies an estimated 90 to 95 percent of the world’s precious opal. This immense production volume is a result of a unique geological history tied closely to the Great Artesian Basin, where the deep weathering of Cretaceous period sedimentary rock created ideal conditions for massive silica mobilization.
The various Australian fields are distinguished by the types of opal they produce. Lightning Ridge in New South Wales is renowned for the highly valued Black Opal, characterized by a dark body tone that provides a dramatic contrast, enhancing the visibility of the play-of-color. Coober Pedy in South Australia is famous for producing lighter body-tone opals, often termed white or milky opal.
Queensland is the origin of Boulder Opal, where the precious opal forms in thin seams within an ironstone or sandstone matrix. This type of opal is often cut with the host rock left intact behind the opal, creating a natural, durable backing. Australian deposits are generally classified as sedimentary opals.
International Sources of Precious Opal
While Australia sets the standard, significant and distinct opal deposits are found across the globe. Ethiopia has emerged as a major producer, particularly with the discovery of Welo opal, which is often volcanic in origin. A distinguishing feature of much Ethiopian Welo opal is its hydrophane nature, meaning it is porous enough to readily absorb water.
When a hydrophane opal absorbs water, its transparency and body color can change significantly, often becoming transparent before returning to its original state upon drying. The Welo opal, discovered in 2008, is typically found in nodules within volcanic ash and is known for its vivid play-of-color.
Mexico is another globally recognized source, primarily known for its vibrant Fire Opal. This material is typically found in deposits within rhyolite rock formations in states like Querétaro and Jalisco. Mexican Fire Opal is named for its body color, which ranges from bright yellow to vivid orange and deep red.
Unlike many precious opals, the value of Mexican Fire Opal is often based purely on this intense body color, and it may not exhibit the characteristic play-of-color. Other countries contribute smaller amounts to the global supply, including the United States (Idaho and Nevada) and Brazil, which has yielded golden fire opal.