Onyx (in the stone industry sense) is a banded calcite or aragonite deposit formed by mineral-rich water flowing through caves and fissures over millions of years. Each band represents a layer of calcium carbonate precipitated from solution — the same process that forms stalactites and stalagmites. Unlike marble, which forms under heat and pressure, onyx forms at low temperatures through slow chemical deposition. This layered structure is what makes onyx translucent: the thin, parallel bands of crystalline calcite transmit light between them rather than scattering it. When backlit, the bands glow at different intensities depending on their thickness and purity, creating the luminous effect that makes onyx unique among natural stones.