Onyx

My first rock sphere was an onyx specimen. The way this specimen is cut makes a circle and a v-shaped band. Based on the info below, onyx is banded linearly while agate's bands are curved.
Here are the facts from about.com's geology guy, Alden.
Onyx is composed of chalcedony, with straight bands of color instead of the curved forms typical of agate. So if true onyx is a banded chalcedony, a marble with the same appearance should be called banded marble instead of onyx marble, and certainly not alabaster, which is not banded at all.
Chalcedony is the special name for quartz with microscopically small crystals. Unlike quartz, chalcedony does not look clear and glassy but translucent and waxy; like quartz it is hardness 7 on the Mohs scale. Unlike quartz it can take on every color imaginable. An even more general term, encompassing quartz, chalcedony and opal, is silica, the compound silicon dioxide (SiO2).
The major rock type that is defined by the presence of chalcedony is chert. Chalcedony also very commonly occurs as a mineral filling veins and openings, like in geodes and thunder eggs.