Obsidian is a type of natural glass formed from rapidly cooled lava. While commonly pictured as deep black, obsidian can display blue or other colors, which is scientifically accurate. Its true color depends on a combination of its base chemical composition and the way light interacts with microscopic structures within the glass. This volcanic material offers a complex study where color is determined by both chemistry and physics.
The Standard Color: Why Obsidian is Usually Black
The typical deep black color of obsidian is a direct result of its chemical makeup. This volcanic glass is rich in silica, but it also contains high levels of mafic components like iron and magnesium oxides. These transition metal oxides are powerful light absorbers across the entire visible spectrum, giving the glass its characteristic dark hue.
The high concentration of these elements means little light passes through the material, making it appear opaque and black. Since the material lacks an ordered crystalline structure, these light-absorbing atoms are distributed uniformly throughout the glass. This uniform distribution ensures the deep, consistent black color.
Optical Illusions: The Phenomenon of Blue and Rainbow Sheen
The striking colors of blue, gold, or rainbow seen in some obsidian are not due to the bulk color of the glass itself. These vivid effects are purely structural, known scientifically as iridescence or a sheen. This phenomenon occurs when light interacts with microscopic inclusions embedded near the surface. These inclusions are often sub-microscopic crystals of magnetite or flattened bubbles of water vapor or gas.
As the lava cooled, these minute structures aligned themselves into parallel layers within the glass matrix. When light enters the obsidian, it reflects off these multiple, closely spaced layers. This process creates thin-film interference, where light waves constructively and destructively interfere. The resulting effect is a vibrant, shifting color display, such as the blue sheen, that changes depending on the viewing angle.
The spacing between these layered inclusions determines the specific wavelength of light that is amplified. For instance, extremely fine and evenly spaced layers preferentially scatter blue light. Rainbow Obsidian showcases a full spectrum of colors due to the varying sizes and spacing of its internal layers. The color is a display of light physics interacting with the glass structure, not the material’s inherent chemistry.
Chemical Inclusions and True Color Variations
True variations in the body color of obsidian, distinct from the optical sheen, arise from chemical impurities and subsequent mineral formation. For instance, the presence of oxidized iron (hematite or limonite) can tint the entire glass matrix. This chemical inclusion is responsible for the deep red and brown hues found in “Mahogany Obsidian,” making the color an inherent part of the glass.
The oxidation state of the iron determines the specific shade, with higher oxidation leading to richer reds and browns. Another common variant is Snowflake Obsidian, where the color variation is caused by the formation of tiny, radial clusters of the mineral cristobalite. These white or grey mineral growths crystallized after the surrounding glass had solidified, standing out sharply against the black matrix.
Green obsidian is a much rarer color variation, typically linked to geological environments where the iron content remains in a reduced state. These colors are uniform throughout the material, proving that the composition of trace elements plays a significant role in the overall appearance of the volcanic glass.
The Amorphous Structure of Volcanic Glass
Obsidian is classified geologically as an extrusive igneous rock, defined by its status as a natural glass. It forms when highly viscous, silica-rich lava, typically of rhyolitic composition, cools extremely rapidly. This swift cooling process, often occurring when lava flows hit water or air, prevents the atoms from arranging themselves into an ordered, crystalline lattice structure.
The resulting material is described as amorphous, meaning it lacks a defined, repeating internal structure. This lack of crystalline grain contributes to the glass’s uniform appearance and its characteristic conchoidal fracture. When struck, the material breaks along smooth, curved surfaces, similar to man-made glass. The high viscosity of the original lava facilitates this glassy structure by inhibiting atomic movement necessary for crystal growth.
The amorphous nature is the fundamental property that allows for the uniform black color and also permits the formation of the microscopic inclusion layers responsible for the optical effects. Since the structure is non-crystalline, light is not scattered by internal grain boundaries. This unique structure defines all of obsidian’s physical properties.