also: サードニクス
noun
sardonyx
A variety of onyx with alternating bands of sard and white chalcedony; used as a gemstone.
サードニックスは古代から印章やカメオに使われてきた。
Sardonyx has been used for seals and cameos since ancient times.
Onyx is a broader term for banded chalcedony; sardonyx specifically has bands of sard (reddish-brown) and white.
Agate is a general term for banded chalcedony; sardonyx is a type of agate with distinct red and white bands.
Loanword from English 'sardonyx', ultimately from Latin 'sardonyx' and Greek 'sardonyx'.