Hydrophane

Hydrophane was a variety of opal that was a rather plain, opaque, ivory or frosty-white color when dried, but when immersed in water it became transparent with a prism-like rainbow iridescence.

Description
Because of this, the semi-precious stone was highly prized by sailors and the aquatic races of Faerûn and used in decorating water-related items. A typical stone had a base value of 50 gp.

Also see

 * Black opal
 * Fire opal
 * Opal
 * Water opal