Onyx is a banded variety of chalcedony. The colors of its bands range from white to almost every color (save some shades, such as purple or blue). Commonly, specimens of onyx contain bands of black and/or white.
Marble rarely contain fossils when you have a rock that has formed through metamorphosis then created through heat and pressure acting on other rocks. Fossils are inherently delicate things and therefore precious little would withstand the conditions which creates marble - and therefore marble hardly ever contains fossils as a direct result of this