The captivating appearance of opal, characterized by its internal flashes of spectral color, known as the play-of-color, has made it a highly desired material for centuries. This phenomenon occurs due to the diffraction of light as it passes through a precisely ordered internal structure of microscopic spheres. As the demand for this unique gemstone has grown, so has the need for a reliable, consistent, and more durable source than traditional mining can provide. This market need led to the development of laboratory-created alternatives, which replicate the opal’s fundamental structure in a controlled environment.
Defining Synthetic Opal
Lab-created opal is correctly identified as a synthetic opal, meaning it shares the same fundamental structural and chemical building blocks as its natural counterpart. Natural opal is an amorphous mineraloid composed of hydrated silica (SiO₂·nH₂O). Synthetic opal, unlike imitation opal made from materials like glass or plastic, is also primarily composed of silica and exhibits the defining play-of-color created by the same optical principle of light diffraction.
The material is often referred to in the trade as Gilson Opal, a name originating from Pierre Gilson, who pioneered the first commercially successful synthesis in the 1970s. Modern variations are often classified by the Gemological Institute of America (GIA) based on their composition. For instance, some are termed “Polymer Impregnated Synthetic Opal,” consisting of approximately 80% silica and 20% stabilizing resin. This confirms the silica structure is present, even if the material contains a polymer to enhance its stability rather than the water found in natural opal.
The Hydrothermal Growth Synthesis Process
The process of creating synthetic opal in a laboratory is a multi-stage method designed to mimic the geological conditions that form the stone over millions of years. This manufacturing technique focuses on the controlled assembly of microscopic silica spheres, which are the material’s structural foundation.
Sphere Preparation
The first step involves the chemical preparation of these spheres, typically using a modified Stöber method where a chemical precursor, such as tetraethyl orthosilicate, is hydrolyzed in an alcoholic solution. This reaction generates monodisperse silica particles, meaning they are highly uniform in size, usually around 300 nanometers in diameter.
Sedimentation and Stacking
The second stage is sedimentation, where the particles are allowed to settle slowly by gravity in a controlled environment. This process can take several months, as the spheres must arrange themselves into a highly ordered, three-dimensional lattice structure. This precise stacking, known as a colloidal crystal array, is necessary because the play-of-color effect relies on the diffraction of light.
Solidification and Stabilization
The third step involves a hydrothermal treatment where the settled structure is subjected to heat and pressure. This treatment cements the silica spheres together, solidifying the fragile precursor material into a robust block. In many modern synthetic opals, the final step involves impregnating the porous silica structure with a polymer resin. This polymer acts as a stabilizing agent, filling the microscopic voids and replacing the natural water content typically found in mined opal.
Structural Differences from Natural Opal
The controlled laboratory process results in a structural uniformity that distinctly sets synthetic opal apart from its natural counterpart. Natural opal forms geologically over vast periods, leading to random variations, inclusions, and an irregular pattern of color flashes. By contrast, the meticulous sedimentation of lab-grown spheres often produces a highly consistent and orderly pattern. Under magnification, this regularity can manifest as a distinct “lizard skin” or columnar structure, where the play-of-color appears in neatly aligned columns or stripes.
A significant chemical difference lies in the stabilizing agent used within the structure’s voids. Natural opal contains a variable percentage of water, which makes it susceptible to “crazing” or cracking if it dehydrates or is exposed to sudden temperature changes. Many commercial lab-created opals replace this unstable water content with a polymer resin, which chemically bonds the silica spheres. This polymer impregnation increases the material’s structural integrity and durability.
While the natural material is generally rated between 5.5 and 6.5 on the Mohs hardness scale, the polymer-impregnated synthetic material is often more resilient to cracking. This enhanced stability is a functional benefit of the synthetic process, as it removes the risk of water loss and resulting internal stress. The precise control over sphere size and packing also allows manufacturers to engineer a more vivid and consistent color display.
Practical Applications and Physical Characteristics
The enhanced physical characteristics of lab-created opal have made it highly desirable for a variety of commercial and artistic applications. Its superior durability and resistance to crazing make it a practical choice for everyday jewelry, including rings, pendants, and bracelets, where natural opal might be too fragile. The consistency of color and pattern achievable through the synthesis process also simplifies the creation of matching jewelry sets or inlays that require uniform color blocks.
The material’s composition, often incorporating a polymer stabilizer, contributes to its improved workability. It is easier to cut, shape, and polish than natural opal, allowing for greater flexibility in design and manufacturing. Furthermore, the ability to produce large, defect-free blocks of the material at a predictable rate makes it a more cost-effective option than rare and expensive high-grade natural stones. This combination of affordability, consistency, and durability has broadened the use of opal to decorative objects, specialized inlays, and other applications.