Sardonyx

Sardonyx was a variety of banded onyx.

Description
Sardonyx was characterized by alternating stripes of carnelian (also known as sard) and white onyx. These semi-precious stones were usually cut cabochon to show off the banded pattern but were also used in cameo work. A typical specimen had a base value of 50 gp.

Powers
Sardonyx had many uses in the magical Art. It was used as a spell component and in creating magic items which affected wisdom. It was reported to have the same properties as the much rarer king's tears except that it was not indestructible. And it was one of the "nine secrets" (gemstones that could become ioun stones if enchanted properly). Wearing or carrying sardonyx could also absorb a minimal amount of damage from a magic missile, but this was a random occurrence and thus not very reliable.