Typically found in Brazil, India, Uruguay, and Pakistan, it is a variety of chalcedony characterized by parallel, straight bands of color (unlike agate, which has curved or concentric bands), often in black and white or monochromatic tones. It forms through the cyclic precipitation of colloidal silica in cavities within volcanic or sedimentary rocks, with successive layers of deposition that give it its banded structure. Much of the black onyx on the market is obtained by artificially dyeing grayish chalcedony. Hardness 6.5–7 on the Mohs scale, density 2.6 g/cm³. It is important to distinguish it from so-called “Mexican onyx” (or “calcareous alabaster”), which is not a variety of quartz but a banded carbonate rock (calcite or aragonite travertine, CaCO₃), with completely different origins and physical properties.

Widely used in jewelry (rings, cameos) and decorative objects since ancient times; highly prized by collectors and artisans for the clarity of its bands, ideal for cameo carving; also used in crystal therapy as a stone associated with strength and protection.