Obsidian is a naturally occurring volcanic glass formed when molten lava cools so rapidly that mineral crystals do not have time to grow. This rapid cooling results in an amorphous structure, leading to its classification as a mineraloid rather than a true mineral. The glass is typically dark, often jet black, and possesses a characteristic conchoidal fracture, meaning it breaks with smooth, curved surfaces and extremely sharp edges. This extrusive igneous rock has been used by humans for millennia to create tools, weapons, and decorative objects.
The Chemical Consistency of Obsidian
Geologically, there is only one fundamental type of obsidian, defined by its high silica content (65% to 80% \(\text{SiO}_2\)) and glassy structure. This volcanic glass is compositionally similar to rhyolite. The molten material is extremely viscous due to the high percentage of silica, which prevents the formation of an ordered crystal lattice.
Obsidian is also characterized by a very low water content, typically less than one percent by weight. Visual differences are caused by trace impurities, micro-inclusions, or trapped gas, allowing the material to be categorized by appearance even though the underlying rock is chemically uniform.
Varieties Based on Crystalline Inclusions
Some varieties of obsidian derive their unique appearance from the presence of partially formed crystals or mineral deposits. Snowflake Obsidian is a well-known example, characterized by white or gray spots that resemble snowflakes against the dark glass. These spots are spherulites, which are radially clustered formations of the mineral cristobalite. These formations result from devitrification, a process where the amorphous glass slowly begins to crystallize over time.
Another distinct variety is the Apache Tear, which occurs as small, rounded nodules of black obsidian. These pebbles are often found embedded in perlite, a hydrated volcanic glass formed when water is absorbed by the surrounding obsidian. The denser obsidian cores resist this hydration, remaining as intact nodules within the softer, altered perlite matrix.
A rarer variety is Macusanite, a volcanic glass found in the Macusani region of Peru. This material is noted for its high clarity and unique inclusions, such as the high-alumina mineral andalusite. Macusanite’s composition, while still high in silica, was once mistaken for a tektite, a glass formed by meteorite impacts.
Varieties Defined by Light and Luster
A second major group of obsidians is distinguished by how light interacts with microscopic structures, creating effects of color and sheen. Rainbow Obsidian exhibits a striking iridescent sheen when viewed under direct light. This iridescence results from thin-film interference caused by layers of nanoscale inclusions, such as magnetite nanoparticles or hedenbergite nanorods, aligned along the lava flow direction. These structures diffract light, splitting it into spectral colors that create the rainbow effect.
Sheen Obsidian, including Gold and Silver varieties, is characterized by a metallic, reflective luster that appears to float just beneath the surface. This chatoyancy, or “cat’s eye” effect, is caused by the inclusion of tiny gas bubbles, specifically water vapor, which are stretched nearly flat as the viscous lava flowed. These micro-tubes act as parallel reflectors that bounce light back to the viewer, producing the distinct shimmer.
Mahogany Obsidian presents a mottled or banded appearance, featuring swirls of dark reddish-brown within the black glass. The color is caused by the presence of iron oxide impurities, specifically hematite, which stained the glass during its formation. These iron-rich areas often follow the flow patterns of the lava, creating the characteristic swirling patterns.
Less Common and Regional Varieties
Many other names for obsidian varieties are descriptive terms based on specific visual characteristics or their geographic source. Velvet or Midnight Obsidian is a trade name used to describe material that is a uniform, exceptionally dark black, often with a smooth, non-reflective surface when polished. This denotes a pure specimen free of the common inclusions that cause sheen or color banding.
Another distinction is between true obsidian and Pitchstone, a closely related volcanic glass. Pitchstone is chemically similar to obsidian but is defined by a significantly higher water content, typically ranging from four to ten percent by weight. This higher water concentration gives Pitchstone a duller, more resinous luster compared to the glassy surface of obsidian.