Hydrophane

Hydrophane is a variety of opal that is a rather plain, opaque, ivory or frosty-white color when dry, but when immersed in water it becomes transparent with a prism-like rainbow iridescence. Because of this, hydrophane was highly prized by sailors and the aquatic races of Faerûn and used in decorating water-related items. A typical specimen had a base value of 50 gp.