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From The Collaborative International Dictionary of English v.0.48:
Onyx \O"nyx\, n. [L., fr. Gr. ? 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]