Serpentine

Serpentine was the name given to a family of related materials which included williamsite, ricolite, verde antique, picrolite, taxoite, bowenite, and sometimes referred to as poor man's jade. Top quality samples of williamsite were classified as semi-precious stones and all other varieties were treated as hardstones sold under the name serpentine stone.

Description
Gem quality serpentine was translucent with an intense green color and could be facet cut or made into cabochons to decorate weapons, armor, and barding or mounted in rings, pendants, and jewelry of all types. A typical specimen had a base value of 50 gp. cabochon

Powers
Carrying, wearing, or being in contact with a gemstone of this type (williamsite) could confer the equivalent of a resist cold and a resist fire to that individual. If more than one serpentine gem was present, only one stone activated per exposure to extreme heat or cold at a time. Each gem could protect against one cold effect and one heat effect before crumbling to useless dust.