Black tourmaline and obsidian, two dark and often confused materials, share a similar deep black appearance that can make distinguishing them a challenge for the untrained eye. Despite their visual similarities, these two substances possess distinct geological origins and physical properties. Understanding these differences provides clear methods for identification, moving beyond mere visual observation to reveal their unique characteristics.
Fundamental Nature
Black tourmaline is a crystalline mineral belonging to the silicate group. It forms deep within the Earth’s crust, typically in igneous and metamorphic rocks, under high heat and pressure or hydrothermal activity. Its internal structure consists of an organized, repeating arrangement of atoms, which gives it a defined crystal lattice.
In contrast, obsidian is not a mineral but a naturally occurring volcanic glass. It originates from molten lava that cools so rapidly upon eruption that mineral crystals do not have time to form. This rapid cooling results in an amorphous structure, meaning its atoms are randomly arranged. This fundamental difference in formation and internal structure underpins many of their varied physical properties.
Visual and Tactile Clues
Both black tourmaline and obsidian exhibit a glassy luster. Obsidian often presents a highly reflective, mirror-like surface, especially when polished. Black tourmaline’s luster, while still glassy, can be more subdued or appear slightly matte, even after polishing. While both are generally opaque black, thin edges of obsidian can sometimes show a degree of translucency when held against a light source. Black tourmaline, conversely, is almost always completely opaque, with no light passing through it.
Their surface textures and markings also differ. Black tourmaline frequently displays characteristic parallel striations, which are fine grooves or lines. These striations can often be felt. Obsidian, due to its amorphous nature, lacks these striations and possesses a smoother, more uniform surface.
When fractured, both materials exhibit a conchoidal fracture, producing shell-like, curved surfaces. Obsidian’s fracture edges are often exceptionally smooth and can be razor-sharp, a sharpness that results from its uniform, amorphous structure allowing cracks to propagate evenly. Black tourmaline’s conchoidal fracture edges, while present, may not be as consistently sharp or perfectly smooth.
Hands-On Identification Methods
A simple hardness test provides a reliable method for distinguishing these materials: black tourmaline is harder than obsidian, rating between 7 and 7.5 on the Mohs scale, while obsidian has a Mohs hardness of approximately 5 to 6. This difference means that black tourmaline can scratch obsidian, but obsidian cannot scratch black tourmaline. A simple scratch test involves attempting to scratch an unpolished area of the unknown specimen with a steel nail (Mohs hardness 5.5) or a piece of quartz (Mohs hardness 7). A steel nail should leave a mark on obsidian but not on black tourmaline.
Density and weight also offer clues. Black tourmaline is generally slightly denser than obsidian. For a similarly sized piece, black tourmaline may feel marginally heavier. Black tourmaline has a specific gravity of around 3.15, whereas obsidian’s specific gravity is approximately 2.4.