Hypersthene

Hypersthene, also known as bronzite, was a gemstone usually cut cabochon.

Description
This semi-precious stone was brown and opaque (sometimes reddish or greenish) and contained silvery spangles. It was rare to find an unflawed piece larger than $1/2$ in (13 mm) across, and as a result the stone yielded small gemstones. A typical stone had a base value of 50 gp.

Powers
Carrying or wearing one of these gemstones made the bearer undetectable by the wyvern watch spell. In common parlance this property was known as wyvern warding.