Green obsidian is a captivating material, a form of volcanic glass much rarer than its common black counterpart. It is not a true mineral because it lacks a crystalline structure, instead solidifying as an amorphous glass. It originates from volcanic eruptions, where specific geological conditions must align for the material to take its distinct emerald or dark green hue. Understanding this material requires exploring the rapid cooling process that defines all obsidian, the subtle chemistry that creates the color, and the geographic locations where this unusual glass can be found. The rarity of genuine green obsidian also leads to frequent misidentification in the commercial market.
The Geological Formation of Obsidian
Obsidian is classified as an igneous rock, produced when molten material cools so quickly that atoms cannot arrange themselves into an ordered crystal lattice. This process requires highly viscous lava, which is rich in silica and classified as felsic or rhyolitic magma. The silica content is usually high, ranging between 65 and 80 percent \(SiO_2\). This high viscosity physically inhibits the diffusion of atoms required for crystal growth.
Rapid quenching, where the lava makes sudden contact with a cooler environment like air or water, locks the atomic structure into a disordered, glass-like arrangement. This results in the characteristic smooth, uniform texture of obsidian. The cooling rate differentiates obsidian from common volcanic rocks like granite or rhyolite, which cool slowly enough to form visible mineral crystals. All obsidian, regardless of color, exhibits a conchoidal fracture, a break pattern that creates smooth, curved surfaces with extremely sharp edges.
The Origin of Green: Chemical Composition and Coloration
The color of any obsidian is determined by trace elements and the state of oxidation within the glass matrix. While the bulk of obsidian is silica, the vibrant green color is caused by trace amounts of transition metals, primarily iron and sometimes chromium. The oxidation state of the iron within the glass is the determining factor.
In most common black obsidian, the dark coloration is caused by tiny crystals of iron oxide, particularly magnetite. For the material to appear green, the iron must be present in a specific valence state, often as ferrous iron (\(Fe^{2+}\)). This ferrous iron absorbs certain wavelengths of light, subtracting colors from the visible spectrum and leaving the green hue. This mechanism is distinct from red obsidian, where the iron is highly oxidized to its ferric state (\(Fe^{3+}\)) and manifests as hematite crystals, producing a reddish-brown color.
The exact shade of green can vary from a deep forest green to an olive tone, depending on the concentration of iron and the presence of other elements like magnesium or chromium. In some cases, the green coloration results from microscopic inclusions of other minerals, called microlites, which scatter light to create the color effect. This subtle variation in chemical composition and oxidation state makes the formation of consistently green obsidian a rare event in volcanic geology.
Natural Deposits and Distinguishing Man-Made Varieties
Genuine natural green obsidian is a geographically limited material, making it highly sought after and often expensive. Significant deposits are found in areas with past rhyolitic volcanic activity.
Natural Deposits
Historically significant deposits are found near Pachuca in Mexico, which are known to yield green specimens. Other verified sources include locations in Guatemala and parts of the western United States, notably California and Oregon. A more recently discovered, though still debated, source has been named Elmerite from Ethiopia.
Distinguishing Varieties
Due to its scarcity, the market is frequently saturated with fraudulent materials marketed as “green obsidian.” Common substitutes include man-made slag glass, bottle glass, or materials like Healerite, which is actually a serpentine variety.
A primary way to distinguish the authentic stone is to look for the presence of inclusions and bubbles. Man-made glass often contains perfectly spherical gas bubbles, a result of the manufacturing process. Conversely, natural obsidian typically has fewer bubbles, which are often stretched or flattened due to the high viscosity of the cooling lava flow.
The color of imitation material is often an unnaturally bright, uniform emerald green. Genuine green obsidian typically presents as a dark, dense, and opaque green, only becoming translucent around the very thin edges. The physical origin and the microscopic structure of the glass are the definitive factors in separating the rare volcanic product from industrial byproducts.