Rainbow obsidian is a natural volcanic glass recognized for a colorful sheen. This material is a variety of the more common black obsidian, but its iridescent quality sets it apart. The rainbow-like display is a phenomenon of structural color, not pigment, resulting from a specialized formation process. Its relative scarcity makes it a sought-after material for collectors and artisans.
The Formation of Standard Obsidian
The base material for rainbow obsidian is standard obsidian, an extrusive igneous rock formed from felsic lava rich in silica. This lava, similar in composition to rhyolite, typically contains a high concentration of silicon dioxide, often exceeding 70% by weight. The high silica content causes the molten material to be extremely viscous.
The formation of the glass structure requires a process called quenching, where the lava cools rapidly upon extrusion from a volcano. This rapid cooling, often occurring at the edges of a lava flow or when lava encounters water or air, prevents the atoms within the melt from organizing into a regular crystalline structure. Instead of forming mineral crystals, the atoms are frozen in place, resulting in an amorphous solid state.
The Mechanism of Iridescence
The colorful display that distinguishes rainbow obsidian is not caused by mineral pigments but by the physical interaction of light with the stone’s internal structure. This structural color is a result of a phenomenon known as thin-film interference. This mechanism is similar to how colors are produced in a soap bubble or an oil slick on water.
The color effect originates from microscopic inclusions trapped within the solidifying glass. These are typically nanometric mineral particles, most often tiny crystals of magnetite (an iron oxide mineral), or sometimes hedenbergite or feldspar rods. These particles are aligned in parallel layers within the obsidian, creating a series of extremely thin, stacked films.
When light waves strike these parallel layers, they are reflected and refracted off the top and bottom surfaces of the films. The light waves then interfere with each other, based on the thickness of the layer and the wavelength of the light. This interference selectively enhances certain wavelengths of light, splitting white light into its component colors and producing the iridescent sheen. The specific colors observed depend on the precise thickness of the inclusion layers and the angle at which the material is viewed.
Specific Geological Requirements and Occurrence
Achieving the iridescent effect requires a set of geological conditions more specific than those needed for standard obsidian formation. While rapid cooling is necessary to form the glass matrix, the formation of rainbow obsidian demands a slightly slower, more controlled cooling phase within the larger flow. This allows the trace minerals, such as iron and magnesium, to precipitate out of the melt as microscopic crystals.
The particles must not only form but also align themselves into the parallel layers necessary to produce the thin-film interference. This alignment occurs while the lava is still flowing and partially viscous, with the movement of the flow stretching the mineral inclusions into organized planes. The specific chemical composition of the rhyolitic lava must also contain the requisite metal oxides, such as iron, copper, and manganese, which feed the formation of the magnetite and other inclusions.
Because this combination of chemical composition, specific cooling rate, and flow dynamics is uncommon, rainbow obsidian is geographically restricted. Significant deposits are found in areas with a history of silica-rich volcanic activity. Major sources in the United States include Glass Buttes in Oregon and certain sites in Nevada. Mexico is also a well-known producer, particularly for varieties that exhibit strong bands of color.