Onyx and obsidian are two distinct natural materials often confused due to their similar dark appearances. While both are used in jewelry and decorative items, they originate from entirely different geological processes and possess unique properties. This article aims to clarify the differences between onyx and obsidian, providing insight into their formation, characteristics, and practical ways to distinguish them.
What is Onyx?
Onyx is a form of chalcedony, which is a microcrystalline variety of quartz. It consists of tiny quartz crystals that have formed together into a compact mass. This mineral is primarily composed of silica (SiO2). Onyx forms in cavities within rocks, often through the slow deposition of silica-rich solutions from groundwater.
The distinctive feature of onyx is its parallel bands of contrasting colors, most commonly black and white, though it can also appear in shades of brown, red, and green. These bands result from variations in the concentration or composition of the silica-rich solutions during its formation. Onyx ranges from translucent to opaque, allowing some light to pass through. It has a waxy luster that can be polished to a high shine and a Mohs hardness of 6.5 to 7, indicating its durability.
What is Obsidian?
Obsidian is a naturally occurring volcanic glass, classified as an igneous rock. It forms when felsic lava, which is rich in silica, cools very rapidly without allowing crystals to grow. This quick cooling occurs when molten lava comes into contact with water, air, or other cool surfaces. The rapid solidification prevents atoms in the molten rock from arranging into an orderly crystalline structure, resulting in an amorphous, glassy texture.
Obsidian is primarily composed of silica (65-80%), with smaller amounts of other elements. It is dark black but can also appear in brown, gray, red, green, or even exhibit a golden sheen due to gas bubbles or other inclusions. Obsidian has a glassy luster and a Mohs hardness of 5 to 6, making it softer than onyx. When fractured, obsidian exhibits a conchoidal fracture, meaning it breaks with smooth, curved, shell-like surfaces that can be sharp.
How They Differ and What They Share
Onyx and obsidian differ fundamentally in their geological origin and internal structure. Onyx, a microcrystalline quartz, forms through slow deposition of silica, while obsidian, a volcanic glass, forms from rapidly cooled lava. This structural difference impacts their fracture patterns: onyx can exhibit a conchoidal fracture but is known for its parallel banding, whereas obsidian consistently displays a conchoidal fracture, producing sharp edges. Onyx is harder (Mohs 6.5-7) than obsidian (Mohs 5-6), making it more resistant to scratches and wear.
Both materials are popular choices for jewelry and decorative objects due to their aesthetic appeal. Historically, both have been valued for their beauty and utility, with obsidian’s sharp edges making it particularly useful for tools and weapons in ancient times.
Simple Ways to Tell Them Apart
Distinguishing between onyx and obsidian can be done by observing a few key characteristics. Visually, onyx often exhibits distinct parallel bands or lines, even if subtle, which may be white or other colors against a black background. Obsidian, on the other hand, has a uniform, solid color and a glassy, shiny appearance without banding.
Holding the material up to a strong light source can also reveal differences. Onyx can appear translucent at its edges, allowing some light to pass through. Obsidian is opaque, but very thin pieces or specific varieties might show slight translucency or a unique sheen like a golden or rainbow effect. When comparing pieces of similar size, onyx feels heavier than obsidian because it is denser.
The feel of the surface can also provide a clue; onyx tends to have a smoother, waxier feel, while obsidian, being volcanic glass, feels sharper along any fractured edges. If a piece is chipped or broken, obsidian will shatter with a sharp, curved, glass-like fracture, whereas onyx breaks in a more granular or chunky manner.