The deep black color and high polish of black onyx and black obsidian often cause confusion, leading many to believe they are the same material. However, these two stones are fundamentally different, created by distinct geological processes and possessing unique internal structures. Understanding these differences reveals they are not interchangeable, but rather two separate materials with unique properties and applications.
How They Are Formed (Geological Origin)
The formation of black obsidian is a rapid process linked to volcanic activity. As an extrusive igneous rock, obsidian is created when silica-rich lava is expelled from a volcano and cools extremely quickly upon contact with air or water. This rapid cooling prevents the atoms from arranging into an ordered, crystalline structure, resulting in a natural glass sometimes referred to as volcanic glass.
Black onyx, in contrast, is the product of a much slower, sedimentary or hydrothermal process. It is a form of chalcedony, which is a microcrystalline quartz. This material forms when silica-rich water seeps into rock cavities and slowly deposits layers of silicon dioxide over millions of years. These alternating layers create the characteristic parallel bands that define onyx, even if they are subtle in a black specimen.
Chemical Structure and Classification
The internal makeup of the two stones distinguishes their mineralogical classification. Black obsidian is categorized as a mineraloid, a naturally occurring substance that lacks a crystalline structure. Its composition is primarily silicon dioxide, but its atoms are arranged randomly, resulting in an amorphous structure. This non-crystalline nature is a direct result of its rapid formation from molten lava.
Black onyx, conversely, is classified as a true mineral and a variety of quartz. It possesses a cryptocrystalline structure, meaning its crystals are too fine to be seen without high magnification. The structure of onyx is composed of silicon and oxygen atoms arranged in a fixed, repeating pattern. Its defining feature is the presence of parallel layers, which classifies it as a banded chalcedony, even when the stone appears uniformly black.
Identifying Physical Differences
The materials’ different internal structures result in several practical, observable differences for identification. Obsidian is significantly softer, rating 5 to 5.5 on the Mohs scale of hardness. Onyx is harder and more durable, rating 6.5 to 7, meaning obsidian is more prone to scratching and wear.
When broken, obsidian displays a distinctive conchoidal fracture, which produces smooth, curved, and shell-like surfaces. This glassy structure allows the edges of a fractured piece to be extremely sharp. Onyx, being a true mineral, breaks with an uneven or splintery fracture, lacking the glassy smoothness of obsidian. Obsidian exhibits a high vitreous, or glassy, luster, whereas onyx typically has a waxy or slightly duller sheen, though it can be polished to a high gloss.
Historical and Modern Applications
The unique physical properties of each stone have dictated their uses throughout history and in modern industry. Obsidian’s glass-like nature and ability to produce an incredibly sharp edge led ancient civilizations, such as the Mayans and Aztecs, to use it extensively for tools and weapons. Today, this property makes obsidian a material of choice for specialized, high-precision surgical scalpel blades.
Onyx, being a harder and more durable material, has traditionally been valued for its ability to withstand intricate carving. Its layered composition made it a preferred material for creating cameo jewelry, seals, and other decorative objects in ancient Roman and Egyptian cultures. Modern applications include jewelry, where its durability allows it to be used in rings and pieces subjected to daily wear, as well as in ornamental architectural features.