Opal is a unique gemstone, classified as a mineraloid rather than a mineral because it lacks a definitive crystalline structure. Its composition is hydrated silica dioxide, meaning it contains a variable amount of water within its structure. The presence of blue opal in nature is an established fact, but this color is often subject to enhancement or outright imitation by manufacturers. Understanding the difference between a natural stone, a laboratory creation, or a composite material is necessary.
Natural Blue Opal Occurrence and Composition
Natural blue opal exists primarily as common opal, which does not exhibit the characteristic internal flashes of color, called “play-of-color,” seen in precious opal. This stone is typically opaque to translucent, valued for its solid body color rather than iridescence. Two recognized natural sources are Peruvian Blue Opal and Owyhee Blue Opal from the United States.
Peruvian Blue Opal, found in the Andes Mountains of Peru, ranges from pastel to intense turquoise-blue or blue-green. Its distinctive color is caused by trace mineral impurities, such as microscopic copper inclusions. Owyhee Blue Opal, mined along the Oregon-Idaho border, displays a soft, sky-like blue hue with a translucent quality.
The Geological Formation of Opal
Opal forms through a geological process called opalization, which involves silica-rich water seeping into cracks, fissures, and voids within the Earth’s crust. As the water slowly evaporates, it leaves behind deposits of hydrated silica spheres. The arrangement of these spheres determines the type of opal that forms.
Precious opal forms when the spheres are of a uniform size and stack in an organized, three-dimensional lattice; light diffracted through this structure creates the vibrant play-of-color. Conversely, common opal forms when the silica spheres are irregular in size and jumbled together in a random arrangement. This disorganized structure prevents the diffraction of light, resulting in a consistent, solid body color. The formation process occurs at relatively low temperatures and can take place in various host rocks, including volcanic and sedimentary types.
Blue Opal Imitations and Synthetics
The market for blue opal contains both synthetic stones and simulants. Synthetic opal, often sold under trade names like Gilson or Kyocera opal, is chemically and structurally similar to natural opal, grown in a laboratory using the same silica spheres. However, the accelerated growth process results in a structure that is often too orderly and uniform compared to a natural stone.
Simulants look like opal but have a different chemical composition, often being made of glass, plastic, or resin. These imitators, such as Slocum Stone, mimic the visual appearance. Natural opal is also frequently treated to enhance its appearance or stability. Treatments include the creation of doublets and triplets, which are composite stones designed to make a thin slice of opal appear larger and more vibrant.
Doublets and Triplets
An opal doublet consists of two layers: a thin slice of natural opal glued to a dark backing, often made of black potch (common opal), plastic, or ironstone. The dark backing enhances the color and brightness of the thin opal layer. An opal triplet adds a third layer, which is a clear, protective cap, typically made of quartz, glass, or plastic, placed over the thin opal slice. This clear dome protects the opal layer and magnifies its color, giving the stone a glassy, domed appearance.
Practical Identification Tips
Distinguishing natural blue opal from its man-made counterparts requires careful visual inspection. A key indicator for a synthetic opal is the visibility of a “lizard skin” or “chicken wire” pattern when the stone is viewed under magnification. This repeating, often uniform pattern is a result of the orderly stacking of silica spheres in the lab-grown material, which rarely occurs in natural opal.
If the stone is a doublet or triplet, looking at the side profile is the most reliable method. A perfectly straight, horizontal line marking the junction where the layers are glued together is a definitive sign of a composite stone. Natural solid opal, even if backed by its host rock, will have a more irregular or organic boundary between the layers.
Additionally, triplets have a glassy, highly polished top surface that reflects light differently than the softer sheen of a solid natural opal. Finally, natural opal often feels cool to the touch and has a higher density, while simulants made of plastic or resin tend to feel warmer and lighter.