Obsidian is a naturally occurring volcanic glass formed when lava cools very quickly. This rapid cooling prevents the atoms from arranging into an ordered, crystalline structure, which is the defining feature of glass. The high silica content of the lava creates high viscosity, inhibiting the growth of mineral crystals as the material solidifies. Obsidian is therefore an amorphous solid, lacking the internal structural organization of true minerals.
Primary Color and Luster
The most common appearance of obsidian is a dense, jet-black color. This dark coloration is primarily due to trace amounts of iron and magnesium oxides within the silica-rich composition. While black is dominant, obsidian can also appear dark brown, dark gray, or occasionally dark green depending on the specific impurities present. Obsidian possesses a high degree of vitreous luster, meaning it has a characteristic glassy sheen. This reflective quality allows a polished surface to act much like a mirror. The material is smooth and hard, typically rating between 5 and 6 on the Mohs hardness scale.
The Unique Way Obsidian Breaks
A defining characteristic of obsidian is the way it breaks, known as conchoidal fracture. This fracture does not follow any flat, pre-existing planes of separation, unlike crystalline minerals. Instead, it creates smooth, curved surfaces that visually resemble the concentric undulations found on a mussel shell. The amorphous structure, lacking internal weaknesses, allows the applied force to radiate outward from the point of impact, resulting in the distinctive curved breakage. Where these shell-like surfaces intersect, they form edges that are exceptionally thin and sharp, often finer than a surgical scalpel.
Notable Patterns and Varieties
Beyond the standard black form, various inclusions and trapped elements create several visually distinct varieties of obsidian. These unique appearances are caused by specific minor components or events during the cooling process.
Snowflake Obsidian
Snowflake Obsidian is the most common patterned variety, appearing as black obsidian dotted with small, radially clustered white or gray spots. These contrasting spots are spherulites, which are tiny crystal formations of the mineral cristobalite that form when the glass partially begins to devitrify.
Rainbow Obsidian
Rainbow Obsidian displays a brilliant, iridescent sheen when polished and viewed under light. This striking effect of banded colors, including gold, green, blue, and purple, is caused by the interference of light waves reflecting off microscopic layers. These layers are composed of oriented nanoparticles of minerals like magnetite or hedenbergite.
Sheen Obsidian
Sheen Obsidian exhibits a metallic golden or silvery reflection just beneath the surface. This visual effect is caused by the reflection of light off countless minuscule gas bubbles that were stretched and flattened during the lava flow.
Mahogany Obsidian
Mahogany Obsidian features a mix of black or dark brown with distinct reddish-brown patches, bands, or swirls. This two-toned pattern results from iron impurities, such as hematite, being incorporated into the glass during its formation. The contrast gives the rock an appearance similar to the grain of mahogany wood.