Obsidian and onyx are frequently encountered in jewelry, decorative objects, and historical artifacts. Their often dark appearance and smooth finishes can lead to confusion, yet they possess distinct geological origins and physical properties. Understanding these differences provides clear guidance for accurate identification.
What is Obsidian?
Obsidian is a naturally occurring volcanic glass. It forms when silica-rich lava, typically rhyolitic in composition, cools very rapidly, preventing the formation of mineral crystals. This rapid quenching results in an amorphous, non-crystalline structure.
While commonly known for its dark, often jet-black color, impurities can give obsidian hues of brown, green, red, or even an iridescent sheen. It typically exhibits a smooth, glassy texture and a highly reflective, vitreous luster.
When obsidian breaks, it displays a characteristic conchoidal fracture, meaning it forms smooth, curved, shell-like surfaces with extremely sharp edges. This unique fracture pattern made obsidian highly valued for cutting tools and weapons by ancient civilizations.
What is Onyx?
Onyx, in contrast, is a type of chalcedony, which is a microcrystalline form of quartz. Its formation involves the deposition of silica-rich solutions in cavities and fissures within rocks, creating layers of tiny quartz crystals. The most recognizable feature of onyx is its distinctive parallel banding, often seen as alternating layers of black and white. Depending on the impurities present during its formation, onyx can also exhibit bands of brown or other colors. Black onyx, a popular variety, is typically opaque and presents a waxy or somewhat dull luster, which can be polished to a higher sheen.
Practical Ways to Tell Them Apart
Several practical characteristics allow for distinguishing between obsidian and onyx.
Luster
Luster offers a primary clue; obsidian inherently possesses a strong, glassy, and reflective shine, whereas onyx typically exhibits a waxy or slightly duller finish. Even when highly polished, onyx may not achieve the same mirror-like reflectivity as obsidian.
Patterns
Observing patterns provides another key differentiator. Onyx is defined by its distinct, often straight and parallel bands or layers of varying colors. While pure black onyx may lack visible banding, most other varieties clearly display this layered structure. Obsidian can sometimes show subtle flow banding or an iridescent effect caused by trapped gas bubbles, but lacks the clear layered structure of onyx.
Translucency
A light test can also be informative. Due to its glassy nature, thin edges of obsidian often exhibit some translucency when held up to a strong light source. Conversely, onyx, particularly the black variety, is generally opaque and will not allow light to pass through.
Fracture
The way each material breaks provides further evidence. Obsidian’s characteristic conchoidal fracture produces smooth, curved surfaces and very sharp edges. Onyx, being crystalline, tends to fracture with a more irregular or granular break.
Hardness
Hardness is a reliable physical property for differentiation. Onyx ranks higher on the Mohs scale of mineral hardness, typically between 6.5 and 7, indicating greater resistance to scratching. Obsidian is softer, with a Mohs hardness ranging from 5 to 5.5. This means a common steel nail, which has a hardness of approximately 5.5, could potentially scratch obsidian but would not scratch onyx.
Feel and Weight
Additionally, obsidian often feels smoother and colder to the touch than onyx due to its glass-like composition. Onyx, being denser, may feel noticeably heavier than a piece of obsidian of the same size.
Common Misidentifications and Final Tips
Obsidian and onyx are frequently confused due to their dark color and similar polished appearance. Careful observation of their unique properties is essential for identification. When attempting to identify a piece, it is helpful to systematically examine its luster, look for any banding patterns, and test its translucency against a light source. While hardness tests can be useful, exercise caution with valuable pieces, as scratching can cause damage. Performing a gentle scratch on an inconspicuous area, if necessary, is advisable. For definitive identification of important or valuable items, consulting with a professional gemologist or mineralogist is recommended.