The rocks which form the Earth, the Moon, and the planets are made up of minerals. Minerals are solid substances composed of atoms having an orderly and regular arrangement. This orderly atomic arrangement is the criterion of the crystalline state and it means also that it is possible to express the composition of a mineral as a chemical formula.
When minerals are free to grow without constraint, they are bounded by crystal faces which are invariably disposed in a regular way such that there is a particular relationship between them n any one mineral species. A crystal is bounded by naturally formed plane faces, and its regular outward shape is an expression of its regular atomic arrangement.
We are familiar with symmetrical objects such as boxes, furniture, and even ourselves. Close inspection of such objects will reveal that they can be symmetrical about a plane such that if the object were to be cut in half along the plane, one half would be the mirror image of the other. The human body is symmetrical externally about a vertical plane arranged from front to back.
On the basis of symmetry, crystals can be grouped into six crystal systems, and can be referred to imaginary reference axes. A seventh crystal system, the trigonal, is recognized by many mineralogists. It has the same set of reference axes as the hexagonal system, but has a vertical three-fold axis of symmetry. These reference axes are chosen so as to be parallel to the edges of the unit cell (the repeat unit of pattern in a crystal structure), and hence they can be regarded as having length. Most minerals occur as aggregates of crystals that rarely show perfect crystal shapes. The form of the aggregate, however, can be useful in identification. There is a close link between the structures of a mineral and its physical properties which are, accordingly, of considerable value in identification. And the optical properties depend on the interaction of light.
Nearly all rocks are composed of minerals, but fine specimens are rare and tend to occur in what is called fissures and other cavities where the crystals have been unobstructed during their growth. Many are obtained from the mineral veins. High temperature fluids deposit minerals in cracks and fissures in rocks and many of these veins, are worked as sources of ore. They frequently contain colorful specimens and good crystals, not only of the commercially valuable ore minerals, but also of the accompanying and economically valueless gangue minerals as well.
It is not always necessary to examine or collect from the veins themselves ~ in many instances, it is dangerous or impossible to do so - for mining activity usually results in dumps of discarded material which, if carefully searched, will often yield good specimens.
Good crystals can often be found lining cavities in rocks of virtually every kind, though particular minerals tend to occur in certain environments. Sometimes weathered-out cavity linings, called geodes, are lined with well shaped crystals, and many fine crystals of Amethyst occur in such associations. Pegmatites, which crystallize from relatively low-temperature, volatile-rich magma (fluids), are another source of crystals and rare minerals that frequently grow to large sizes.
Collectors will find some crystals and minerals that are difficult to identify. They are urged to become acquainted with those that are displayed in many national and other museums. Time spent in this way will be amply repaid, not only in terms of identification, but also in becoming more deeply involved in the study of the natural symmetry and natural history.
