Algae

Algae was an ornamental stone found in Faerûn that was used as decoration on baldrics, belts, and as an inlay on fine furniture.

Description
Algae was a variety of quartz similar to banded agate but the bands were wavy and usually found in shades of dark brown. These gemstones were usually cut cabochon or sliced, and polished to a glassy finish to bring out the design. A typical stone had a base value of 10 gp.

Powers
A being in direct contact with algae, or wearing it, was more likely to resist polymorphing effects caused by spells, psionic attacks, and other arcane or divine shape-altering enchantments. This resistance was conferred whether the subject was willing or not.