sardonyx

Sardonyx
Photo courtesy of Scott Papper, GIA GG, AJP, CM (NAJA), Global Gemology & Appraisals
Variety:  Sardonyx
Species:  Chalcedony
Mohs Hardness:  5 to 6 out of 10
Color:  Banded variety of chalcedony that has sard or carnelian with alternating bands of white onyx, black onyx, or both.
RI:  1.535 to 1.539 (1.53 to 1.54 spot reading)
Birefringence:  Usually not detectable
SR/DR/AGG:  AGG
Spectrum:  Not diagnostic
Fluorescence:  Generally inert
SG:  2.60 (+0.10/-0.05)
Routine Treatments:  None

sardonyx facts

Victorian antique Scottish dirk brooch featuring carved sardonyx set between hand chased sterling silver end caps (with replacement pin and safety catch)
Photo courtesy of Scott Papper, GIA GG, AJP, CM (NAJA), Global Gemology & Appraisals

Sardonyx is a banded variety of chalcedony.

While agate has curved bands, sardonyx has parallel bands.

Sardonyx is composed of either sard or carnelian with alternating bands of white onyx, black onyx, or both.

Sardonyx was popular for hard stone cameo carvings.
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Sardonyx Gallery