Obsidian is a naturally occurring volcanic glass, formed when lava extruded from a volcano cools rapidly with minimal crystal growth. This rapid cooling process prevents the formation of large mineral crystals, resulting in a unique material with a smooth, uniform texture. Its striking appearance and distinctive properties have fascinated people for centuries. Obsidian forms an igneous rock, rich in silica, which contributes to its high viscosity during formation.
The Dominant Color
The most common and widely recognized color of obsidian is a deep, opaque black. This dark appearance results from the presence of microscopic inclusions of iron and magnesium within the glass. Its amorphous structure, which means it lacks a crystalline arrangement, contributes to its inability to transmit light.
As light strikes the surface of black obsidian, it is absorbed rather than allowed to pass through, creating its characteristic dark hue. This process gives black obsidian its characteristic vitreous luster, reflecting light like polished glass.
Beyond Black: Other Obsidian Hues
Beyond its prevalent black form, obsidian exhibits a captivating array of other colors and patterns. Rainbow obsidian, for instance, displays an iridescent sheen that shimmers with a spectrum of colors, including vibrant greens, purples, reds, and golds. This colorful display becomes particularly noticeable when the obsidian is cut and polished.
Another distinct variety is snowflake obsidian, characterized by white or gray patterns resembling delicate snowflakes against a dark background. Mahogany obsidian features reddish-brown streaks or patches, often swirled within a black base. Sheen obsidian, whether golden or silvery, exhibits reflective qualities that appear to glow from within the stone.
These less common hues add to the material’s visual diversity, making each piece unique. The various colors and patterns are influenced by specific conditions during the lava’s cooling process and the presence of different inclusions.
The Science Behind Obsidian’s Colors
Other colors arise from different inclusions and optical phenomena. The reddish-brown tones in mahogany obsidian, for example, are due to the presence of iron oxide, such as hematite or limonite, within the glass. These mineral inclusions are introduced into the molten lava during the eruption process.
Sheen obsidian’s golden or silvery reflections are caused by patterns of tiny gas bubbles or microscopic mineral inclusions, often magnetite or hematite, aligned within the flowing lava before it solidified. These inclusions interact with light, creating the shimmering effect.
Snowflake obsidian’s distinctive white patterns are formed by spherulites, which are radial clusters of cristobalite crystals, a type of quartz. These crystals develop when the lava cools slightly slower, allowing for some localized crystal growth. In rainbow obsidian, the vibrant iridescence is a result of light diffraction and thin-film interference caused by nanoscopic inclusions of magnetite or other mineral particles, which are arranged in layers.