Onyx is formed through a geological process involving the deposition of silica minerals, primarily chalcedony, in layered bands. The formation of onyx typically occurs in cavities within volcanic rocks or in the fissures and voids of sedimentary rocks. Here’s a breakdown of the formation process:

1. Silica-Rich Waters:
Onyx forms from silica-rich water, which seeps into cracks and voids in volcanic or sedimentary rocks. This water contains dissolved silica, often originating from the weathering of silicate minerals.

2. Deposition of Chalcedony:
As the silica-rich water evaporates or cools, it begins to deposit layers of chalcedony, a microcrystalline variety of quartz. Chalcedony is deposited in layers over time, with each layer representing a new phase of silica-rich water entering the cavity or fissure.

3. Formation of Bands:
The characteristic bands of onyx form as a result of variations in the composition of the silica-rich water. These variations can include differences in the presence of trace elements or impurities, which cause the chalcedony to crystallize in different colors or shades. The bands are usually parallel and can vary in thickness.

Black and White Bands: The most common onyx exhibits alternating black and white bands. The black bands are usually caused by the presence of organic material or iron and manganese oxides, while the white bands are pure chalcedony.
Other Colors: Onyx can also display bands in other colors, such as red, brown, or green, depending on the impurities present during the formation process.
4. Time and Pressure:
Over long periods, the repeated deposition of chalcedony layers under pressure forms the solid, banded structure of onyx. The process can take millions of years, depending on the conditions.

5. Final Stages:
In the final stages, the onyx is completely formed and may remain within the host rock until it is exposed through erosion or mined by humans.

Summary:
Onyx is a banded variety of chalcedony, formed from silica-rich waters that deposit layers of chalcedony in cavities or fissures within rocks. The distinctive parallel bands of color are created by variations in the chemical composition of the silica solution, with each layer forming over a long period. The resulting stone is a striking and durable material prized for its beauty and used in jewelry and decorative items.

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