Opal is a unique and captivating gemstone, classified as a mineraloid because it lacks the rigid crystalline structure of true minerals. It is a hydrated amorphous form of silica, composed of silicon dioxide combined with water, which can range from 3% to 21% by weight. The gem’s fame comes from the internal flashes of spectral color, a phenomenon known as “play-of-color.” This distinctive feature is rare, stemming from the specific and prolonged geological conditions required for its creation.
The Geological Prerequisites for Opal Formation
The formation of opal requires a precise combination of high silica concentration, water, and deep weathering over vast stretches of time. The process begins when water, often slightly acidic, seeps through the earth and dissolves minute particles of silica from surrounding rocks, typically sandstone. This silica-rich solution then travels down into cracks, faults, and voids in the underlying rock layers.
Over millions of years, as the water slowly evaporates, it leaves behind a deposit of hydrated silica gel. This process is exceedingly slow, estimated to take five million years to form a layer just one centimeter thick. The difference between common opal (often milky or opaque) and precious opal lies in the internal arrangement of microscopic silica spheres.
In common opal, or “potch,” the spheres are randomly stacked, resulting in a uniform appearance without color. Precious opal features silica spheres of uniform size arranged in an orderly, three-dimensional grid. When white light passes through this structure, it is diffracted, splitting into the visible spectrum and producing the dynamic play-of-color that defines the gemstone.
Global Hubs of Precious Opal Discovery
The vast majority of the world’s precious opal has historically come from Australia, supplying over 90% of the global market. Australian deposits formed in the sedimentary rocks of the Great Artesian Basin, which was an inland sea during the Cretaceous period. The three main types of Australian opal are defined by their unique host rock and appearance:
- Coober Pedy, in South Australia, is the source of light opal and crystal opal, which feature a translucent to opaque light body tone.
- Lightning Ridge in New South Wales is the exclusive source of the highly valued black opal, where carbon and iron oxide impurities create a dark body tone that dramatically intensifies the play-of-color.
- Boulder opal from Queensland forms in thin seams within ironstone concretions, resulting in a durable gem where the dark host rock is left attached to the opal layer.
A major new source emerged in 2008 with the discovery of Welo opal in Ethiopia’s Wollo Province, rapidly becoming a significant competitor. Unlike the sedimentary origin of Australian opals, Ethiopian deposits are of volcanic origin, forming in nodules within layers of weathered rhyolite. This variety is known as hydrophane opal, describing its unique ability to absorb water, which temporarily changes the gem’s transparency and play-of-color.
Regional Sources and Distinct Varieties
Mexico is internationally recognized for its stunning Fire Opal, prized for its vibrant body color, ranging from bright yellow to deep orange and red. This color is not due to light diffraction but to trace amounts of iron oxides acting as coloring agents. Mexican opals, such as those from Querétaro, are often cut en cabochon or left partially embedded in their volcanic host material, a style known as Cantera opal. Another Mexican variety is Water Opal, a colorless, transparent stone that may exhibit a faint bluish or golden internal sheen.
The United States also contains notable opal deposits that are regionally significant. Northern Nevada’s Virgin Valley is famous for producing high-quality precious black opal, often found as opalized wood where silica has replaced ancient organic material. Idaho’s Spencer Mine is known for thin veins of precious opal that are frequently manufactured into opal triplets, a composite gem with a protective cap and dark backing.