Sardonyx is a variety of microcrystalline quartz chalcedony (kal-SED-oh-nee), also known as Black Line Agate, is a dramatic black stone banded with crisp white lines, translucent bands and the occasional venture into the brown and cream and is found all over the world.
It's a stone of strength and protection. Metaphysically, it is believed to improve memory and increase analytical talents. It was said to be used on the breast plates of Roman soldiers for this reason.
Mohs hardness - 7.