The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sardonyx
    n 1: an onyx characterized by parallel layers of sard and a
         different colored mineral

Easton's 1897 Bible Dictionary:

Sardonyx
   (Rev. 21:20), a species of the carnelian combining the sard and
   the onyx, having three layers of opaque spots or stripes on a
   transparent red basis. Like the sardine, it is a variety of the
   chalcedony.