Beautiful Gems

If the supply of the crystalline fluid is not even on all faces of a growing crystal, owing perhaps to currents within the mother liquor, the resultant crystal is to some extent distorted: certain crystal faces are abnormally large, while others are comparatively small. Fluorspar, for example, whose usual crystal shape is a cube, may produce elongated four-sided prisms (Plate 42). Distorted crystals often take on a shape which suggests a symmetry-system other than that to which the mineral actually belongs.
SKELETAL CRYSTALS. If the edges and corners of a crystal grow at a much greater rate than the intervening faces, the resultant structure is a mesh or lattice which is known as a skeletal crystal. The best known skeletal crystals are snowflakes, and the nearest approach among natural minerals is the skeletal quartz illustrated in Plate 75. The so-called dendrites, which are brown or black “skeletal structures,  are formed  from ‘iron- or manganese-bearing solutions which penetrated along thin cracks or planes in the rock and crystallised only after the solution had largely dried up. In shape, dendrites often resemble ferns, mosses, trees or the ice-ferns on window panes.

It can be proved mathematically that among the innumerable possible crystal shapes there are only 32 distinct types of symmetry, known as symmetry classes. These fall into six major groups called crystal systems, the cubic, tetragonal, hexagonal (with the trigonal subsystem), orthorhombic, monoclinic and triclinic systems. The most common minerals of the earth’s crust, the feldspars, crystallise in the monoclinic and triclinic systems.

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).