Beautiful Gems

Minerals which have been allowed to develop their own crystalline shape are called euhedral or idiomorphic. Those minerals which crystallised in the spaces between earlier formations and are bounded not by their own but by pre-existing surfaces, are said to be anhedral or allotriomorphic. Yet others which have replaced a pre-existing crystal and thus taken on its shape, are termed xenomorphic. Completely euhedral crystal forms are not common, as most crystals grew up from a base and so are only partially idiomorphic (Plate 6).

Most crystals found in nature are to some degree imperfect, as they usually stand on a surface, such as the wall of a druse, from which their growth started. The crystals which are completely perfect are the ‘floating’ crystals which grew in a soft medium, as, for instance, gypsum crystals in soft clay. The uneven supply of solution during crystallisation leads to distortions and generally the perfect growth of the crystal is prevented by the presence of adjoining crystals or the host rock itself. Due to such external influences, the shape of natural crystals frequently falls far short of the ideal shape. Apart from obvious structural distortions, there are other defects, such as small inclusions of other minerals, which may locally disrupt the orientation of the crystal lattice. Crystals of fairly large size are thus hardly ever completely uniform. Usually they consist of a mosaic of small blocks of homogeneous crystals which are variously orientated and may frequently interlock.

MINERALS AND CRYSTALS. The word mineral is derived from the Latin minare, to mine, and was originally used to include all rocks which were obtained through mining. Nowadays, the word mineral is used to describe those materials of the earth’s crust formed by the inorganic processes of nature, which have a definite chemical composition and whose constituent atoms are arranged in a consistent pattern. Rocks, on the other hand, are defined as aggregates of one or more minerals. Most rock types are composed of several different minerals; of those made up of only one mineral, the best known is marble, which is formed of interlocking crystals of calcite.
The name crystal, which was first used to describe rock-crystal, the clear, transparent form of quartz, has its root in the Greek word krystallos, meaning ice. In the early Middle Ages and before, it was thought that rock-crystal was made of ice which had been so intensely frozen that it could never again melt.

Crystals are now defined as bodies which have definite geometrical forms bounded by flat faces and straight edges. The crystalline state, of course, is not only found among natural minerals, but also occurs in many artificially produced substances, such as refined salt and sugar, and metals, and among waste products of living organisms. The crystalline form is really the external expression of the internal atomic structure of the mineral. The constituent atoms of any given mineral species are arranged in a definite geometric pattern known as the crystal lattice. This lattice determines the mineral’s physical properties, which include hardness, specific gravity, the ease and direction of splitting, and the effects on light transmitted through it .Though the internal structure of any given mineral is always the same, the shape of the crystal is often imperfect, since the growth of crystals in nature is always dependent on the space available to them. The name crystal, however, is used not only for the perfect form, but also for any part of a crystalline substance which has at least some crystal faces and edges. Not all minerals are crystalline. Some are completely structureless and are termed amorphous, but these are relatively rare among natural minerals, the best known being opal and amber. Amorphous minerals are never bounded by flat surfaces, and never have an ordered lattice structure.

They usually have roundish, indefinite shapes (Plate 15). Under certain conditions, an amorphous mineral may, in the course of time, become finely crystalline. Opal, for instance, tends in due course to change into the minutely crystalline {cryptocrystalline} mineral chalcedony.