How is synthetic opal primarily produced?

Synthetic opal is primarily produced by the precipitation of microscopic silica spheres. This method mimics the natural formation of opal, where silica molecules are deposited in a liquid state.

During the process, a solution containing silica is allowed to evaporate or cool, resulting in the crystallization of silica into tiny spheres. These spheres then arrange themselves in a specific structure, which is crucial for creating the unique play-of-color that opal is known for. The size and arrangement of these silica spheres contribute significantly to the optical properties of opal, allowing it to exhibit a diverse range of colors and patterns, characteristic of both natural and synthetic specimens.

Other methods mentioned, like using melted silica or hydrothermal synthesis, are typical for other types of gemstones or materials, but they do not accurately describe the predominant technique for producing synthetic opal. Chemical vapor deposition is also not relevant in the context of opal production; it is more commonly used in the creation of diamonds or coatings rather than in replicating the silica structure of opal.