Snowflake Obsidian is a striking type of volcanic glass distinguished by its dark, often black, base color speckled with distinctive white or gray inclusions. These markings give the stone its name, resembling frozen flakes of snow set against a night sky. The material is a form of obsidian, an igneous rock, and its unique texture results from specific geological conditions following the initial cooling of molten rock.
Formation of Volcanic Glass
The story of Snowflake Obsidian begins with the formation of its base material, which is standard obsidian. This process requires the rapid cooling of highly viscous, high-silica lava, typically of a rhyolitic composition, which contains 70% or more silicon dioxide (SiO₂). The high silica content causes the molten rock to be extremely thick, inhibiting the movement of atoms necessary to form an ordered structure.
When this viscous lava is extruded from a volcano and cools very quickly, such as when it flows into a body of water or air, the atoms do not have time to arrange themselves into a crystalline lattice. This lack of a regular, repeating atomic structure results in a natural glass, which geologists refer to as an amorphous solid. This glassy state gives obsidian its characteristic smooth, vitreous luster and its ability to fracture with extremely sharp, curved edges.
The resulting black glass is chemically uniform but structurally unstable compared to fully crystallized rock like granite or rhyolite. The black color of the glass matrix is usually due to trace amounts of iron and magnesium oxides. This initial rapid cooling stage creates the dark foundation before the mechanisms responsible for the white “snowflakes” take effect.
The Process of Devitrification
The defining “snowflakes” within the black glass result from a secondary, slower process known as devitrification, the partial crystallization of the unstable volcanic glass. This process begins after the initial rapid cooling and can occur over extended periods deep within the solidified obsidian flow. The amorphous glass structure slowly begins to revert toward a more stable, crystalline state.
The white inclusions are tiny, radially structured clusters of mineral crystals called spherulites. These spherulites are composed primarily of cristobalite, a high-temperature polymorph of quartz. The formation of these radial clusters is a solid-state reaction, meaning it occurs within the already-cooled, solid glass.
Devitrification is often initiated and accelerated by the presence of water or other volatile components remaining within the glass matrix. Even trace amounts of water can act as a catalyst, reducing the energy barrier required for the silica molecules to rearrange themselves. The spherulites grow outward from a central point, or nucleus, creating the circular, snowflake-like pattern that stands out against the dark glass.
The size and density of the white spherulites depend on factors like water content, the cooling rate of the surrounding glass, and the time elapsed since formation. When the devitrification process is complete, the rock loses its glassy appearance and becomes a fine-grained, crystalline rock. Snowflake Obsidian represents an intermediate stage, where the process has only partially occurred, preserving the contrast between the unreacted glass and the new crystalline growths.
Composition and Geological Settings
Snowflake Obsidian is chemically similar to granite and rhyolite, defined by its high concentration of silicon dioxide. The composition is generally greater than 70% silica, which is required for the high viscosity lava that forms the glass. While the black glass is amorphous, the white spherulites are crystalline, composed of cristobalite.
The material exhibits a Mohs hardness ranging from 5 to 6, which is relatively durable but prone to scratching by harder minerals. This hardness is consistent with glass and mineraloids rather than fully crystallized rock. The black color results from minute inclusions of iron oxide minerals, such as magnetite, dispersed throughout the matrix.
This specific variety of obsidian is found in geological settings associated with recent or current volcanic activity that produced rhyolitic lava flows. Locations worldwide include areas in the Western United States, such as Utah, as well as parts of Argentina and Mexico. Snowflake Obsidian often forms near the margins of these lava flows, where cooling was rapid enough to form glass but conditions allowed for the subsequent devitrification process to begin. This geological context provides the unique thermal and chemical environment required for the partial crystallization that creates its signature pattern.