marble | Definition, Types, Uses, Facts | Britannica
Jan 24, 2020· They are, in the strict sense, neither marble nor onyx, for true onyx is a banded chalcedony composed largely of silicon dioxide. Onyx marble was the "alabaster" of the ancients, but alabaster is now defined as gypsum, a calcium sulfate rock. These marbles are usually brown or yellow because of the presence of iron oxide.