Hydrophane

noun

1. An opal that is almost opaque when dry but transparent when wet.
2. A partly translucent whitish or light-colored variety of opal, which absorbs water upon immersion and then becomes transparent. Also called oculus mundi.
3. A semitranslucent variety of opal that becomes translucent or transparent on immersion in water.
4. A semitranslucent variety of opal that becomes translucent or transparent on immersion in water.