The second of August’s birthstones is sardonyx. This is a very underrated stone in my opinion.
Sardonyx is a variety of Agate with parallel bands of white (or black) contrasting with the brownish-red (sard) body color. Because of its layered nature sardonyx stones have been and are ideal mediums for cameo, intaglio and relief carving.
Agate in turn is a variety of chalcedony which is a variety of quartz. It has a mohs hardness of 6.5-7.
Kathleen Marino MA, GG, AJP, NAJA