Obsidian is a naturally occurring volcanic glass that forms when silica-rich lava cools rapidly with minimal crystal growth. While commonly perceived as uniform jet-black, this volcanic material exhibits a surprising array of colors and spectacular visual effects. The spectrum extends far beyond simple dark shades, encompassing reds, greens, and iridescent sheens. The differences in appearance are not due to pigments but rather to the presence of minute impurities, trapped gases, and structural phenomena.
The Basic Spectrum of Obsidian
The most common form, black obsidian, is not pure glass but owes its deep, opaque color to the presence of trace elements. This coloration is primarily driven by iron and other transition elements dispersed throughout the amorphous silica structure. Specifically, the black hue results from nanoinclusions of magnetite, a form of iron oxide, which absorb light uniformly.
The base palette extends to foundational colors like brown, gray, and mahogany. Mahogany obsidian, for example, is characterized by reddish-brown streaks or mottled patches against a black or dark brown base.
These warmer colors arise from the oxidation of iron within the lava, often hematite, which occurs during the flow before final solidification. The resulting color is a solid body color, unlike the optical effects seen in specialty varieties, and the distinct patterns reflect the varying degrees of iron oxidation and flow direction. Green and blue or yellow hues can also appear, though they are less common and stem from other trace mineral impurities.
Mechanisms Driving Obsidian Coloration
The diverse visual properties of obsidian are governed by three primary geological mechanisms: trace elements, microscopic vesicles, and partial crystallization. Trace elements, such as iron, are dissolved within the glass matrix and are responsible for the base body color, including black and the various shades of brown and red. The concentration and oxidation state of the iron determine the depth and type of the hue.
A second mechanism involves microscopic gas bubbles, known as vesicles, trapped during the rapid cooling phase. These tiny bubbles, often water vapor, can be aligned in layers due to the movement of the flowing lava. When light strikes these layers, it is scattered or reflected, creating a silvery or golden luster, known as sheen.
The third mechanism, partial crystallization, creates some of the most distinctive patterns. If the cooling process is slightly prolonged, the silica molecules begin to organize themselves. This process, called devitrification, leads to the formation of tiny, radially clustered crystals, such as cristobalite. These crystalline formations interrupt the amorphous glass structure, yielding opaque white or gray patterns.
Specialty and Iridescent Varieties
Beyond the solid colors, several varieties of obsidian display spectacular effects driven by light interaction. Sheen obsidian, which includes gold and silver varieties, exhibits a metallic reflection that seems to glow from within the stone.
This effect is caused by light reflecting off numerous, aligned, microscopic gas bubbles or minute inclusions of minerals like magnetite or hematite. Silver sheen obsidian is attributed to trapped water vapor bubbles, while gold sheen comes from aligned iron-rich inclusions.
Rainbow Obsidian
Rainbow obsidian is the most visually stunning variety, displaying a full spectrum of colors when viewed under direct light. The brilliant iridescence is a result of thin-film interference. This occurs because the obsidian contains parallel, nanoscale layers of microscopic inclusions, often magnetite nanoparticles. Light reflecting off these numerous thin films interferes with itself, splitting the white light into its component colors and creating the vibrant, shifting rainbow effect.
Snowflake Obsidian
Snowflake obsidian is characterized by a pattern of white or gray spots that resemble falling snow. This variety is an example of partial devitrification where spherulites of cristobalite have formed within the black glass matrix. These white crystalline clusters contrast sharply with the dark obsidian, providing a textural and visual break from the glassy background. The size and density of the cristobalite spherulites determine the appearance of the “snowflakes,” resulting in unique and distinctive patterns.