A variety of opal, which has the property of becoming transparent on immersion in water. It is also called oculus mundi. We must be careful to immerse them only in pure water, and to withdraw them whenever they have acquired their full transparency. If we neglect these precautions, the pores will soon become filled with earthy particles, deposited from the water, and the hydrophane will cease to exhibit this curious property, and will remain always more or less opaque.