Source : Webster's Revised Unabridged Dictionary (1913)

Sardonyx \Sar"do*nyx\, n. [L., fr. Gr. ?. See {Sard}, and
   {Onyx}.] (Min.)
   A variety of onyx consisting of sard and white chalcedony in
   alternate layers.

Source : WordNet®

sardonyx
     n : an onyx characterized by parallel layers of sard and a
         different colored mineral