Obsidian is a naturally occurring solid admired for its deep, glossy appearance, yet it is not classified as a crystal. Geologically, this material is known as volcanic glass, a type of igneous rock formed directly from molten lava. Obsidian is best described as a mineraloid because it lacks the ordered internal atomic structure that defines true minerals and crystals.
The Defining Feature of Obsidian
The core reason obsidian is not a crystal lies in its rapid formation process, which results in an amorphous structure. Obsidian forms when highly viscous, felsic lava, which is rich in silica (often 70% or more), cools extremely quickly. This rapid quenching typically occurs when the lava flows into a body of water or is exposed to the cold atmosphere.
Rapid cooling prevents the atoms within the molten material from organizing into a regular, repeating three-dimensional pattern. Instead, the atoms are “frozen” in a disordered, random arrangement characteristic of glass. If this lava cooled slowly, the atoms would align, resulting in fully crystallized rock like granite or rhyolite. The lack of internal order makes obsidian fundamentally different from true crystalline solids. This geological distinction classifies obsidian as a natural glass.
What Makes a True Crystal?
A true crystal, or crystalline solid, is defined by an orderly, repeating internal atomic arrangement known as a crystal lattice. Materials like quartz, halite (table salt), or diamond are classic examples that possess this structured internal geometry.
Crystal formation requires a slow, steady environment that allows atoms to bond and stack in their most stable configuration. This internal order gives crystals specific properties, such as a sharp, single melting point and the tendency to break along flat, predictable planes, known as cleavage. Obsidian, conversely, softens gradually over a range of temperatures, a characteristic shared with other amorphous materials.
Distinctive Characteristics and Varieties
The amorphous structure of obsidian dictates its most recognizable physical properties, especially its unique fracture pattern. When obsidian breaks, it exhibits a conchoidal fracture, meaning the surface curves and ripples like the inside of a seashell. This breakage is common in non-crystalline solids and allows obsidian to form edges sharper than surgical steel.
Historically, this sharpness made obsidian a prized material for crafting tools, arrowheads, and weapons across ancient cultures. The appearance of obsidian is characterized by a high, glassy luster, or vitreous shine, reflecting light much like manufactured glass.
While most commonly jet black, obsidian occurs in several varieties caused by impurities or microscopic inclusions.
Varieties of Obsidian
Snowflake Obsidian features white, blotchy patterns caused by tiny, radially clustered crystals of the mineral cristobalite. Mahogany Obsidian contains reddish-brown streaks or patches resulting from iron oxide impurities, such as hematite.
The iridescence of Rainbow Obsidian is due to structural coloration, not crystal formation. This effect is created by aligned, sub-microscopic nanoparticles, such as magnetite, or minute bubbles from the original lava flow. These inclusions interfere with light waves, splitting them into a shimmering, rainbow-like sheen.