Sardonyx

Sardonyx is a banded variety of chalcedony that combines layers of sard (brownish-red chalcedony colored by iron oxide) with layers of onyx (black or white chalcedony). It forms in cavities within volcanic rocks where silica-rich solutions deposit alternating layers over long periods.

Each band represents a change in the chemical composition or conditions of the depositing solution. Iron-rich solutions create the reddish-brown sard layers, while purer silica produces the white or translucent bands. The alternating conditions might reflect seasonal changes in groundwater chemistry or episodic volcanic activity.

Sardonyx has been prized since antiquity. Roman soldiers wore sardonyx rings and talismans carved with images of Mars, believing the stone gave courage in battle. It was one of the most popular stones for cameo and intaglio carving because the contrasting layers allowed artisans to carve figures in white relief against a dark background.

Sardonyx is identified by its straight, parallel bands of reddish-brown (sard) alternating with white or black layers. The banding is typically more regular and linear than in agate. At 7 on the Mohs scale, it scratches glass easily.

Distinguish from banded agate (curved bands, usually lacking the red-brown sard component), carnelian (uniform color, no banding), and tiger eye (chatoyant, different formation). If the brown bands are more orange than reddish-brown, the stone may be classified as carnelian agate rather than sardonyx.