Samarskite

Samarskite was a semi-precious gem used for mourning jewelry or to adorn black ceremonial dress.

Description
Samarskite was a rare-earth mineral, hard and dense, with a velvet-black metallic luster, usually cut cabochon. A typical specimen had a base value of 50 gp.

Powers
One or more samarskite gems, carried or worn on the body, was known to protect against undead by reducing the severity of their attacks by a small amount.