Onyx is a variety of chalcedony, like gate and jasper.
Onyx is always banded with black onyx, containing white bands, being the most abundant form. Black onyx is often dyed to create a homogenous black color.
Black onyx with white bands is always opaque and never lets light pass through, however there are semi-transparent (translucent) forms of onyx that exhibit colorful bands. These forms of onyx are much more precious than normal black onyx.
Translucent forms of onyx are often mistaken for agate and are marketed and sold accordingly.