Opals are prized for their internal flashes of color known as play-of-color. The question of whether opals are “natural” is complex, as the market contains stones ranging from those fully formed by the earth to those entirely created in a laboratory. An opal is scientifically defined as hydrated amorphous silica, meaning it is silicon dioxide with a water content that lacks the rigid crystal structure of traditional minerals. Understanding the various origins and treatments of this gemstone is necessary for any buyer looking to determine the authenticity and value of a stone.
The Geological Origin of Opals
Natural opals are born from a solution of silica and water, a process that unfolds over millions of years deep within the earth. As silica-rich water seeps through the ground, it deposits this solution into voids and cracks within the host rock. When the water slowly evaporates, it leaves behind a gel of hydrated silicon dioxide.
For precious opal to form, the silica must solidify into microscopic spheres that settle and arrange themselves into a highly ordered, three-dimensional lattice. This precise arrangement causes light to diffract, creating the distinctive spectral flashes of color that define precious opal. Natural opals are primarily found in ancient sedimentary deposits, such as those in Australia’s Great Artesian Basin, or in volcanic environments, like the rhyolite rock of Ethiopia.
Manufactured Opals
Opals that did not originate in the earth fall into two main categories: synthetic opals and opal simulants. Synthetic opals, such as Gilson opal, are lab-created materials that share the same chemical composition (hydrated silica) and internal structure as natural opal. They are grown by reproducing the natural sedimentation process, but over a period of months rather than millennia.
Opal simulants imitate the look of opal but possess a different chemical makeup and structure. These imitations may be made from materials like glass, plastic, or resin, often with colored foil or iridescent flakes embedded to mimic the color play. Some simulants are polymer-impregnated materials that contain no water and are chemically distinct from genuine opal.
Enhancing Natural Opals
A third category includes natural opals that have been permanently altered to enhance their appearance, durability, or color.
Sugar/Acid Treatment
One common method is the sugar/acid treatment, typically applied to porous matrix opal. The stone is soaked in a sugar solution and then immersed in acid, which carbonizes the sugar within the opal’s pores. This process darkens the body tone to imitate the look of more valuable black opal.
Smoking
Another technique is smoking, where porous hydrophane opals are exposed to heat and smoke. This process deposits soot within the pores, creating a darker background that makes the inherent colors appear more vivid.
Stabilization
Natural opals may also undergo polymer impregnation or oiling to fill small fractures and stabilize the stone. This makes the opal more resistant to crazing or cracking. These treatments must be disclosed to the buyer.
Distinguishing Natural from Man-Made
The most reliable way for a buyer to spot a manufactured or treated opal is to examine the pattern of the play-of-color. Synthetic opals often display a highly ordered, unnatural appearance, sometimes described as a “lizard skin” or “chicken wire” pattern. This is due to the uniform size and perfect stacking of the lab-grown silica spheres.
Natural opals, in contrast, feature an irregular, fluid, and unique pattern of color flashes, with no two stones being identical. If viewed from the side, a synthetic opal often reveals a columnar structure where the color layers appear neatly stacked in vertical lines. Consumers should also look for signs of assembly, such as the perfectly straight join line visible in doublets or triplets. Finally, an overly consistent or plasticky appearance, or the presence of black spots under magnification, can indicate a sugar/acid or smoking treatment.