on·yx (?n??ks)n. A chalcedony that occurs in bands of different colors and is used as a gemstone, especially in cameos and intaglios.[Middle English onix, from Old French, from Latin onyx, from Greek onux, nail, onyx; see nogh- in Indo-European roots.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.onyx (??n?ks) n1. (Minerals) a variety of chalcedony with alternating black and white parallel bands, used as a gemstone. Formula: SiO22. (Minerals) a compact variety of calcite used as an ornamental stone; onyx marble. Formula: CaCO3[C13: from Latin from Greek: fingernail (so called from its veined appearance)]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014on?yx (??n ?ks, ?o? n?ks) n. 1. a variety of chalcedony having straight parallel bands of alternating colors. 2. (not in technical use) an unbanded chalcedony dyed for ornamental purposes. 3. black, esp. a pure or jet black. adj. 4. black, esp. jet black. [1250?1300; Middle English onix