Commonly, specimens of onyx contain bands of black and/or white. Onyx, as a descriptive term, has also been applied to parallel banded varieties of alabaster, marble, obsidian and opal, and misleadingly to materials with contorted banding, such as "Cave Onyx" and "Mexican Onyx".
Nearly all beads referred to as “black onyx” are color enhanced. This centuries-old process creates a permanent, consistent black color and does not reduce the value of the beads. Black onyx is a color-treated form of grey chalcedony, usually sourced from Asia. True Onyx is banded white and black agate.