NODULAR AND KIDNEY-SHAPED MINERALS
Rounded or kidney-shaped minerals with a smooth, polished-looking surface usually start their life as small masses of glutinous mineral-gel. These gels contain minute particles of mineral matter; in the process of drying up they become gradually harder and in many cases acquire a definite crystalline atomic structure before finally solidifying. Of the minerals which originated as gels, only a few are still amorphous. Opal (Plate 15) is the best known of these. Minerals which were originally gels, but became crystalline in the course of time, are recognised by their smooth, rounded or kidney-like (reniform) shape and their concentrically layered or radically fibrous internal structure. Apart from reniform and hotryoidal (grape-like) masses, those minerals formed from gels may be elongated and thus look like stalactites (Plates 11, 48).