Onyx is a type of banded stone used decoratively in jewelry and architecture for centuries. The term “onyx” describes two materials that, despite their similar appearance and parallel banding patterns, are entirely different in chemical composition and geological origin. Understanding what onyx is made of requires recognizing this distinction between the mineralogical and commercial forms. Both materials feature distinct, often parallel bands of color, but they are derived from two separate classes of minerals.
The Chemical Makeup of Silica Onyx
True mineralogical onyx is a variety of chalcedony, a microcrystalline form of quartz. The chemical compound that makes up silica onyx is silicon dioxide (\(text{SiO}_2\)). This structure is cryptocrystalline, meaning the individual quartz crystals are too fine to be seen without a microscope.
This form of onyx is characterized by straight, parallel bands of contrasting colors, typically black and white, though impurities can cause colors like red or brown. The \(text{SiO}_2\) molecules form a three-dimensional network solid where each silicon atom is bonded to four oxygen atoms, resulting in a durable and hard material. This silicon dioxide structure places it in the same mineral family as amethyst and agate.
The Chemical Makeup of Calcite Onyx
The material most frequently encountered in architectural and decorative applications is chemically distinct from the mineralogical form. This commercial stone is primarily composed of calcium carbonate (\(text{CaCO}_3\)). This type of stone is more accurately described as onyx marble or banded calcite.
The structure of this stone is geologically closer to limestone, travertine, or marble, all of which are carbonate rocks. The wide range of colors—such as green, honey, brown, and white—results from impurities like iron oxides incorporated during its formation.
How the Different Types of Onyx Form
The formation processes for the two types of onyx are fundamentally different, explaining their distinct chemical makeup. Silica onyx forms where silica-rich fluids are present, often associated with volcanic activity or hydrothermal veins. As these fluids deposit silica inside rock cavities, sequential layers build up over time, creating the characteristic straight, parallel bands. Changes in the chemical composition or concentration of the silica solutions during this rhythmic deposition lead to the alternating color bands.
Calcite onyx, or banded calcite, is a sedimentary deposit that forms in cool water environments, most commonly in caves. It is created when calcium carbonate precipitates out of a water solution, such as water dripping from a cave ceiling to form stalactites and stalagmites. This precipitation process is also seen in hot springs and other surface environments, resulting in the layering of \(text{CaCO}_3\) that gives the stone its banded appearance.
Identifying the Physical Differences
The contrasting chemical compositions of silica onyx (\(text{SiO}_2\)) and calcite onyx (\(text{CaCO}_3\)) result in measurable physical differences used for identification. Silica onyx has a hardness of 6.5 to 7 on the Mohs scale, making it hard and scratch-resistant. Calcite onyx is significantly softer, rating only a 3 on the Mohs scale, meaning it can be easily scratched by a metal nail.
Another difference is translucency, which is noticeable in architectural applications. Calcite onyx is often more translucent than the mineral form, allowing light to pass through and making it a popular choice for backlit panels. A definitive test involves a reaction to acid: calcite onyx (calcium carbonate) will effervesce or bubble when exposed to a weak acid, while the silicon dioxide of silica onyx will not react.