In correct usage, onyx refers to a black and white banded variety of agate, or sometimes a monochromatic agate with dark and light bands (brown and white for example) – but traditionally the name was reserved for black and white banded agate, and brown varieties named Sardonyx. In recent times the name has become confused with other banded materials, in particular banded calcite formed in cave systems such as the material found in Mexico and Pakistan and often carved, in fact the majority of carved material that are available today are made from this carbonate rock. Black onyx used in jewellery is dyed agate.
From a Victorian mourning jewellery perspective, black onyx usually feature gold and seed pearl mounts. It is colder than heavier than jet with a more glassy lustre.