Obsidian and onyx are often confused due to their similar dark, polished appearance. However, they originate from distinct geological processes and possess unique properties. This article explores the characteristics that set them apart.
What is Obsidian?
Obsidian is a naturally occurring volcanic glass, formed when felsic lava cools very rapidly. Rapid cooling prevents crystal formation, resulting in an amorphous structure. It is an igneous rock, rich in silica, typically exceeding 70% by weight.
The most common color for obsidian is black, but it can also appear brown, green, or display iridescent sheens due to tiny gas bubbles or inclusions. When fractured, obsidian exhibits a characteristic conchoidal break, producing smooth, curved surfaces and exceptionally sharp edges. Its hardness ranges from 5 to 6 on the Mohs scale. Ancient civilizations used obsidian for tools, arrowheads, and surgical blades. It remains popular in jewelry and decorative objects today.
What is Onyx?
Onyx is a banded form of chalcedony, a cryptocrystalline variety of quartz. It consists of tightly intergrown microscopic quartz crystals. Onyx typically forms from the deposition of silica-rich solutions in rock cavities, often in volcanic regions or within caves, where water evaporates over time, leaving behind layers of minerals.
Onyx is defined by its distinct parallel bands of different colors, though black onyx is often solid-colored and frequently dyed to achieve its uniform dark hue. The bands arise from variations in mineral content or solution chemistry during its formation. Onyx generally has a waxy or vitreous luster and can range from translucent to opaque. It is harder than obsidian, typically measuring 6.5 to 7 on the Mohs scale. Onyx has been historically used for carvings, cameos, and various decorative items.
Distinguishing Obsidian from Onyx
The fundamental differences between obsidian and onyx stem from their geological origins and internal structures. Obsidian is a volcanic glass with an amorphous structure, while onyx is a crystalline form of quartz.
Obsidian has a uniform, glassy texture, often black, with potential iridescence. Onyx is characterized by distinct parallel bands, though solid black varieties exist. Both can be opaque, but thin pieces may show translucency.
Obsidian (5-6 Mohs) is softer than onyx (6.5-7 Mohs). Obsidian exhibits a distinct conchoidal fracture, producing very sharp edges. Onyx typically has an uneven or splintery fracture, not as sharp as obsidian’s.
Despite their similar dark, polished appearance, especially in solid black varieties, key differences allow for accurate identification. Examining for banding, luster (glassy vs. waxy), and relative hardness helps differentiate them.