Hydrophane

Hydrophane was a variety of opal that was highly prized by sailors and the aquatic races of Faerûn because its true beauty was only visible when wet.

Description
This semi-precious stone was typically used for inlay work or cut cabochon for jewelry. It was a rather plain, opaque, ivory or frosty-white color when dry, but when immersed in water it became transparent with a prism-like rainbow iridescence. A typical stone had a base value of 50 gp.

Powers
Hydrophane was used in various potions and magic items related to water, especially those that allowed control over water elementals or water breathing. If used as an additional material component for the color spray spell, it decreased the chance that targets would resist the spell.