The difference between natural opal, formed in the earth, and laboratory-created opal rests entirely on its origin. Opal is chemically defined as a hydrated form of silica, composed of silicon dioxide and a variable amount of water. This unique composition allows for the spectacular optical effect known as “play-of-color” that distinguishes precious opal. While natural opals take millions of years to develop, modern science has replicated this process, yielding materials chemically and structurally identical to their mined counterparts.
The Origin of Natural Opal
Natural opal forms when silica-rich water seeps into cracks and voids within the earth’s crust. As the water slowly evaporates over immense spans of time, it leaves behind a deposit of silica gel. This process occurs in sedimentary environments, such as Australia’s ancient inland seas, or in volcanic settings, like those producing Ethiopian hydrophane opals.
Internal Structure
The defining characteristic of precious opal is its internal structure, composed of microscopic silica spheres arranged in an orderly, three-dimensional array. These spheres act as a natural diffraction grating. When white light enters the stone, it bends around these uniform spheres, splitting into spectral colors that shift and flash as the opal is moved. The size of the spheres determines the color seen, with larger spheres producing reds and oranges, while smaller ones yield blues and violets.
The Creation of Synthetic Opal
The term “synthetic opal” refers to a material manufactured in a laboratory that shares the same chemical composition and structure as its natural equivalent. This process, pioneered by scientist Pierre Gilson in the 1970s, replicates natural formation conditions in an accelerated and controlled environment. Synthetic opal is created by precisely controlling the growth and settling of silica spheres into the required ordered lattice structure. The spheres are suspended in a solution, allowed to settle into a uniform array, and then hardened. This technique ensures the material possesses the same light-diffracting mechanism—Bragg diffraction—that gives natural opal its color play.
Opal Simulants and Treated Materials
It is important to distinguish synthetic opals from simulants and treated opals. An opal simulant is a material that visually resembles opal but possesses a different chemical makeup and internal structure. Examples include glass, plastic-based resins, and products like Slocum Stone. These materials mimic shifting colors but do not rely on the diffraction of light through a silica sphere lattice.
Treated Opals
Treated opals are genuine, natural stones enhanced to improve their appearance, typically by darkening the body color. Common treatments involve processes like the sugar/acid method or smoke treatment, which infuse the porous opal with carbon. This darkening intensifies the contrast and makes the play-of-color appear more vibrant, often mimicking the look of valuable natural black opal.
Identifying Natural Versus Man-Made Opal
Distinguishing between natural and synthetic opal often relies on examining the uniformity and pattern of the color play under magnification. Natural opals display an irregular, fluid, and chaotic pattern of color patches that never repeat precisely. They may also contain minor natural inclusions, such as sand or host rock, which are absent in lab-grown counterparts.
Synthetic Characteristics
Synthetic opals, being products of controlled growth, frequently exhibit a highly consistent and repetitive color pattern. A common identifying feature is the “lizard skin” or “chicken wire” pattern, a honeycomb arrangement visible under magnification due to the perfectly ordered silica spheres. When viewed from the side, a synthetic opal may also reveal a columnar structure, with colors stacked in neat, even layers resulting from the manufacturing process. Additionally, synthetic opals may feel lighter than natural stones and often lack the same level of fluorescence under ultraviolet light.