Black onyx is a popular gemstone recognized globally for its deep, uniform black color and its association with classic elegance in jewelry and decorative carvings. This striking appearance has cemented its place in adornment for centuries. Its solid, dark hue often leads to questions about its fundamental physical properties, specifically concerning how light interacts with the stone. Understanding whether black onyx is translucent or opaque requires defining gemstone clarity.
Defining Translucency and Opacity in Gemstones
Gemstone clarity describes how much light is able to pass through a material, categorized primarily into three types. A stone is considered transparent if light passes through clearly and with minimal obstruction, allowing a person to see images distinctly through the material. Clear glass or a high-quality diamond are common examples of transparent materials.
Translucent stones allow light to pass through them, but the light is diffused or scattered, making it impossible to see a clear image on the other side. This effect is similar to looking through frosted glass. Conversely, opaque materials do not allow any light to pass through the substance, instead absorbing or reflecting all light that hits their surface. An opaque stone presents as dense and solid.
The Mineral Structure of Black Onyx
Black onyx is fundamentally an opaque stone, a property largely determined by its mineral composition and common commercial preparation. The stone is a variety of chalcedony, which is a microcrystalline form of quartz composed primarily of silicon dioxide (\(\text{SiO}_2\)). Its structure consists of extremely fine, layered intergrowths of quartz and moganite, giving it a dense structure that resists the passage of light.
While onyx naturally occurs in banded forms of white, brown, and black, solid black onyx in its pure, untreated state is exceedingly rare. The vast majority of the jet-black material seen in the marketplace today is actually a gray or lighter-colored chalcedony that has been permanently treated.
This process involves soaking the stone in a sugar solution, allowing the liquid to penetrate the minute pores of the quartz structure. The stone is then treated with a strong acid, such as sulfuric acid, which carbonizes the absorbed sugar, turning it into a stable, deep black pigment. This treatment saturates the stone with carbon, enhancing its opacity and creating the uniform black color for which it is prized.
The term “onyx” is sometimes mistakenly applied to banded, translucent calcite materials, which are chemically different and should not be confused with the opaque chalcedony variety.
Practical Ways to Identify Black Onyx
Consumers can use several simple methods to determine if a stone is genuine black onyx or a common imitation like black glass, plastic, or obsidian.
Temperature Test
Genuine onyx, being a dense mineral, will feel cool to the touch and will warm up very slowly when held. Conversely, plastic or glass imitations quickly adjust to body temperature, losing that initial coolness.
Hardness Test
The stone’s Mohs hardness is between 6.5 and 7. This means onyx is hard enough to scratch ordinary glass, but it cannot be easily scratched by a household item like a steel knife.
Visual Inspection
A genuine black onyx should appear uniformly colored and completely opaque. If held up to a bright light source, no light should pass through the body of the stone. If the edges appear translucent or if tiny air bubbles are visible just beneath the polished surface, the item is likely a glass imitation.