Obsidian and onyx are dark, often opaque stones that can appear similar at first glance. Despite their superficial resemblance, these materials possess distinct geological origins and unique physical characteristics.
Understanding Obsidian
Obsidian forms as a natural glass when felsic lava rapidly cools with minimal crystal growth. This rapid solidification prevents crystal formation, resulting in an amorphous material. It typically features a distinctive glassy luster. Edges of fractured obsidian are notably sharp, a characteristic that made it valuable for tools and weapons in ancient civilizations.
While most commonly black, obsidian can also exhibit variations like mahogany (brown streaks) or snowflake (white, star-like patterns of cristobalite crystals). Its texture is typically uniform. When held up to a light source, thin edges of obsidian often show a degree of translucence.
Understanding Onyx
Onyx is a microcrystalline quartz. Unlike obsidian, onyx has a crystalline structure, though its crystals are too small to be seen without magnification. It commonly forms in volcanic rock cavities, where silica-rich water deposits layers. This layered formation is a defining characteristic, often visible as parallel bands.
Onyx typically has a waxy or duller luster compared to obsidian’s glass-like sheen. While black onyx is popular, the stone can also feature white, brown, or red bands. Even in seemingly solid black onyx, faint banding or a slightly uneven texture may be discernible. Onyx is generally more opaque than obsidian.
Key Differences in Appearance and Properties
Luster is a primary distinguishing feature: obsidian consistently displays a bright, glassy sheen, while onyx typically exhibits a softer, waxy, or duller luster. In terms of transparency, obsidian can show translucence along its thin edges when held against a strong light source, whereas onyx is generally opaque.
Fracture patterns also provide a clear distinction. Obsidian is known for its conchoidal fracture, meaning it breaks with smooth, curved surfaces resembling a seashell. Onyx, being a crystalline material, tends to fracture unevenly or granularly. Regarding color uniformity, obsidian is often uniformly black or shows subtle, flowing patterns like swirls, while onyx, even in black varieties, can reveal subtle parallel banding or layers.
Obsidian is softer than onyx. On the Mohs scale, obsidian typically ranks between 5 and 5.5, making it susceptible to scratching. Onyx, being a form of quartz, has a Mohs hardness of 7, making it significantly more resistant to scratching.
Simple Tests for Identification
A simple light test differentiates the two stones. Hold the stone up to a bright light source and examine its thin edges. If the edges appear somewhat translucent, it is likely obsidian. Onyx, conversely, will generally remain opaque.
Another practical test involves assessing its resistance to scratching. Since onyx is harder than obsidian, a steel nail or a piece of glass (which has a hardness of about 5.5) can be used cautiously. Gently scratch an inconspicuous area of the stone; obsidian may show a slight scratch mark, while onyx will likely remain unmarked. Always perform this test on a hidden part of the stone to avoid damage.
Visual inspection for luster and any banding is also effective. Obsidian has a reflective, glass-like surface and uniform color or flowing patterns. Onyx typically has a less reflective, waxy feel, and may show subtle parallel lines or texture variations.