Sardonyx

from WordNet (r) 3.0 (2006)

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

from 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]

from 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.

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