onyx

n. [ L., fr. Gr. &unr_; a claw, finger nail, a veined gem. See Nail, and cf. Onycha. ] 1. (Min.) Chalcedony in parallel layers of different shades of color. It is used for making cameos, the figure being cut in one layer with the next as a ground. [ 1913 Webster ]

2. Same as Mexican onyx. [ PJC ]

3. A deep jet-black color. [ PJC ]

Onyx marble,
Mexican onyx, a banded variety of marble or calcium carbonate resembling onyx. It is obtained from Mexico in various colors ranging from white to deep black. [ 1913 Webster +PJC ]