HYDROPHANÆ, in natural history: A genus of semi-pellucid gems, composed of crystal and earth; the latter ingredient being in large proportion, and mixed imperfectly, as in the chalcidony; and giving a general cloudiness or mistiness to the stone; but of such an imperfect and irregular admixture, as not to be capable of a polish like the chalcidony; and appearing of a dusky and foul surface till thrown into water, in which they become lucid and in some degree transparent, either in part or totally; also changing their colour, which returns to them on their being taken out of the water. To this genus belongs the oculus belli of authors, or whitish grey hydrophanes variegated with yellow, and with a black central nucleus; and the oculus mundi, or lapis mutabilis, which is likewise a whitish grey kind without veins.