Rainbow obsidian is a captivating form of naturally occurring volcanic glass, distinguished by a striking internal iridescence that flashes with spectral colors. Like all obsidian, it forms from the intense heat and rapid cooling of molten lava. This variety possesses a unique characteristic that gives it a vibrant, rainbow-like sheen, leading people to question its availability and worth compared to more common volcanic glass. Understanding the specific geological conditions necessary for its formation and the physics behind its color clarifies its standing in the world of gemstones and minerals.
The Geological Origins of Rainbow Obsidian
The formation of rainbow obsidian begins with the swift chilling of silica-rich, or felsic, lava extruded from a volcano. This rapid cooling prevents atoms from arranging into a crystal structure, resulting in a glass-like amorphous material.
The unique step for the rainbow variety involves the incorporation of microscopic particles into the cooling glass. These inclusions are typically nanoparticles of iron oxides, such as magnetite, or nanorods of minerals like hedenbergite, which become trapped in the glass matrix.
For the characteristic iridescence to appear, these nanoscale inclusions must align themselves in parallel layers during the lava flow and cooling. This precise orientation is an uncommon occurrence, dependent on the dynamics of the viscous lava flow just before it solidifies.
What Causes the Iridescent Color?
The brilliant, multicolored sheen of rainbow obsidian results from a physical light phenomenon known as thin-film interference. This is the same effect that creates the fleeting colors seen in soap bubbles or oil slicks.
When white light enters the polished surface, it interacts with the parallel layers of microscopic inclusions. Light waves reflect off both the top and bottom surfaces of these layers, causing them to travel slightly different distances before exiting the stone.
This difference in travel distance causes the light waves to interfere with each other, either constructively or destructively. Constructive interference enhances certain wavelengths (colors), while destructive interference cancels out others. Since the distance between these internal layers is comparable to the wavelength of visible light, the result is a separation of colors into the spectrum, creating the dynamic, shifting iridescence known as the “schiller” effect. This play of color is angle-dependent, meaning the hue and intensity change as the stone is rotated.
Assessing the Rarity and Availability
The rarity of rainbow obsidian must be addressed in two contexts: geological occurrence and commercial availability. Geologically, the material is less common than ordinary black obsidian. Its formation requires the specific combination of rapidly cooling felsic lava, nanoparticle inclusion, and perfect alignment, making it a specialized find.
Despite these requirements, rainbow obsidian is not considered a precious gemstone because it is reliably sourced from several global locations. Major deposits are found in areas with a history of rhyolitic eruptions, notably Mexico, which is a primary commercial source. Specimens are also yielded in the western United States, particularly Oregon and California.
For the consumer, the material is readily available through mineral dealers, jewelers, and online markets, meaning it is not prohibitively scarce. While its price is higher than common black obsidian, reflecting its complex formation, the consistent supply ensures it is not difficult to acquire for collectors or jewelry use.
Factors Determining Quality and Market Value
Since rainbow obsidian is commercially available, its market value is determined by the aesthetic quality of the individual piece rather than scarcity. Several factors influence its price:
- Intensity and Coverage: The single most important factor is the intensity and coverage of the iridescent flash, or schiller. Specimens that exhibit a strong, vibrant flash across a large portion of the stone command a higher price.
- Color Variety: The variety and number of colors visible in the schiller also influence value. Pieces that display a full spectrum of colors—including vibrant purples, greens, blues, and golds—are generally more highly valued than those that only show one or two muted hues.
- Clarity: Clarity is another consideration, as the presence of cloudiness, known as devitrification, can dull the iridescent effect and reduce the stone’s appeal.
- Size: The size of the piece is a factor, particularly for collectors, as larger pieces with a complete, vibrant color arc are difficult to find and make impressive display specimens.
- Cut and Polish: The quality of the cut and polish is taken into account, as a skilled lapidary can maximize the visibility and brilliance of the internal rainbow effect, optimizing the stone for jewelry or ornamental use.