Onyx and obsidian are two dark stones often mistaken for one another due to their shared deep, opaque appearance. They are fundamentally distinct materials formed through entirely different geological processes. Distinguishing between them requires understanding their unique composition, visual characteristics, and specific structural properties. Examining these factors can reliably determine whether a specimen is the banded quartz mineral, onyx, or the natural volcanic glass, obsidian.
Defining the Stones: Composition and Formation
The core difference between these materials lies in their internal structure and origin. Onyx is a form of chalcedony, a microcrystalline variety of quartz, meaning it is composed of tiny, interlocked silica (\(\text{SiO}_2\)) crystals. This structure forms through a slower process, typically involving the deposition of silica from water in rock cavities, often in sedimentary or metamorphic environments. Onyx is a true mineral, exhibiting a crystalline structure.
Obsidian, conversely, is not a mineral but a naturally occurring volcanic glass. It forms when silica-rich molten lava cools so rapidly that atoms cannot arrange themselves into a regular crystal lattice. This results in an amorphous structure, lacking the geometric pattern characteristic of true minerals. This rapid cooling igneous origin means obsidian is essentially a solid solution, contrasting sharply with the crystalline nature of onyx.
Key Visual Indicators: Color, Banding, and Texture
Observable visual differences provide the most immediate clues for identification. Obsidian is characterized by a deep, consistent black or dark color, often displaying a highly glassy or vitreous appearance. If a thin edge is held up to strong light, it may show slight translucency or a smoky quality, a common trait of glass. Some specimens can also exhibit unique internal features, such as the white spots of “snowflake” obsidian or the shimmering effect of “sheen” obsidian.
Onyx, especially the black variety used in jewelry, tends to be more uniformly opaque, blocking light completely. True onyx, which is typically a banded chalcedony, possesses distinct parallel layers or bands of color, even if a piece is dyed solid black. While a solid black appearance is common in finished onyx pieces, the potential for subtle layering within the stone remains a structural indicator. The surface texture of onyx often feels more mineral or slightly waxy to the touch, sometimes with a slightly duller finish compared to the intensely reflective surface of obsidian.
Structural Tests: Hardness, Luster, and Fracture
The distinct compositions of the two materials translate into measurable differences in their physical properties. Onyx is significantly harder than obsidian, generally rating between 6.5 and 7 on the Mohs scale of mineral hardness. This means onyx is more resistant to scratching and wear, making it a relatively durable choice for everyday use in jewelry. Obsidian, being a glass, is softer and more fragile, ranking lower on the scale at about 5 to 5.5.
The way each material reflects light, known as its luster, also differs markedly. Obsidian possesses a highly distinct, bright, and reflective vitreous luster, a direct result of its glassy, non-crystalline nature. Onyx, while it can be polished to a sheen, often displays a slightly less intense luster, which is sometimes described as waxy or silky, due to its microcrystalline structure. The most definitive structural test involves observing how the material breaks. Obsidian exhibits a conchoidal fracture, breaking like thick glass to produce smooth, curved, shell-like surfaces with razor-sharp edges. Onyx, due to its crystalline composition, typically shows a less predictable, more granular, or uneven fracture pattern.