Beautiful Gems

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.

The number of crystal forms which any one mineral can produce varies enormously. Some minerals never produce any crystal with shaped faces, and some ateonly rarely found in crystalline form or produce only simple crystals. There are others, again, such as topaz, garnet, potash-feldspar, galena, tetrahedrite, pyrite and copper pyrites, which can give rise to a large number of crystalline shapes. The minerals of hydrothermal origin, that is, those which crystallised from dilute solutions at moderately high temperatures, can occur in a particularly large variety of crystal shapes. Barytes, for example, has over 200 distinct crystal forms; the mineral with the largest number is calcite. Several hundred distinct face positions relative to the crystallographic axes have been observed in this mineral, and the combinations of these faces can produce several thousand separate forms. It is quite common to find one mineral in the same deposit, or even specimen, occurring in several crystal forms.

The actual shape of a mineral is determined by its environment during growth. As in the case of a living being, whose inherited characteristics are to some extent modified by its environment, the shape of a crystal depends not solely on its atomic structure but also on such environmental factors as the temperature, confining pressure and concentration of the crystalline fluid, interference from adjacent crystals and, last but not least, the speed of crystallisation.

CRYSTAL GROWTH. The growth of a crystal starts with a nucleus and proceeds by the deposition of layers of the same substance on its surface. This contrasts with the growth of living cells which takes place internally. In a growing crystal the rate of deposition on all faces is usually constant, so that its actual shape does not change during growth. In some cases, however, certain faces grow at a greater rate than others, and the crystal continually changes its shape during growth. If during crystallisation the material deposited on the crystal faces varies in colour or opacity, the completed crystal will have a zoned structure, by means of which its growth pattern can now be studied (Plate 74). There is usually a very gradual change in the composition, concentration, pressure and temperature of the solution in which crystals are growing, and a change in colour or opacity within a crystal is thus usually best seen in those which have taken a long time to form. In this way certain long prismatic minerals are formed, whose ends show a variation in colour or opacity. Such long, pencil-like crystals are commonly found in druses. They are often opaque at their base, becoming gradually clearer upwards until at their apex they are completely transparent (Plates 2, 6). If the supply of the mother liquor is cut off completely, crystal growth will cease. In the flat, tabular aggregates shown in PL. 49, for instance, the growth of all the numerous needle-like crystals came to an end at the same time.

This process, which is known as isomorphous substitution, results in the minerals concerned having a variable chemical composition, and the divisions between mineral species in such an isomorphous series become purely arbitrary. Usually only the most important members of such a series have separate names. The important rock-forming mineral olivine (Plate 17), for instance, forms an isomorphous series whose end members are the magnesium-rich forsterite and the iron-rich fayalite. The soda-lime feldspar series, which is collectively known as plagioclase, forms one third of the earth’s solid crust. This series has as its two end members the sodium-feldspar albite and the calcium-feldspar anorthite (Plate 3). When a mineral species contains varieties which have a different colour or crystal shape from the normal, they are usually given separate names. Amethyst (Plate 2), for instance, which has a purple hue, due to the presence of minute quantities of iron or manganese, is a variety of the mineral quartz; and emerald (Plate 25), whose green colour is due to traces of chromium, is one of the three main colour varieties of the mineral beryl. The colour varieties of the minerals classed as precious stones have been given an unnecessarily large number of separate names, which has led to much confusion. Some substances form several varieties of identical composition, but with slightly different crystalline structure, the variety formed depending on the physical conditions at the time of crystallisation. Potassium feldspar, for instance, crystallises as microcline in many deep-seated igneous rocks, as amazonite in pegmatites, as orthoclase in the cavities (druses) within granites, as sanidine in lavas and as adularia (Plate 76) in certain mineral veins.

There are various chemical compounds which can crystallise into more than one type of lattice. These substances thus give rise to more than one mineral. Calcium carbonate, for instance, forms the two distinct minerals calcite and aragonite, and is thus said to be dimorphous. Another well-known dimorphous substance is iron sulphide, which can crystallise either as pyrite or marcasite (Plate 44). Titanium oxide forms three minerals: rutile, anatase and brookite; and another trimorphous mineral is the aluminium silicate which forms the minerals andalusite (Plate 65), kyanite and sillimanite. So the actual mineral species is determined by the atomic structure as well as by the chemical composition of any given substance. In some crystal lattices certain atoms of a given element can be partially or completely replaced by atoms of another element which are of similar ionic size and have the same electric charge. The substitution of atoms does not greatly alter the structure of the crystal lattice, but changes the chemical composition of the mineral.

The authors and publishers are greatly indebted to the museums and owners of collections who have kindly lent the specimens reproduced in this volume, and who have given assistance in many other ways. Particular thanks are due to Dr E. Jorg of Karlsruhe (National Museum of Natural History), Dr O. Griitter of Basle (Museum of Natural History and Ethnology), Dr M. Schnetter of Freiburg im Breisgau (Town Collection), Dr M. Griinenfelder of Zurich (Mineralogical Collection, Eidgenossische Technische Hochschule) and Dr H. Dachs of Munich (Institute of Crystallography and Mineralogy, Munich University).

Mr Georg O.Wild of Idar-Oberstein has very kindly permitted us to photograph specimens of minerals belonging to his firm, as also has Mr W. Finck of Freiburg im Breisgau, who has provided specimens both from his private collection and that of his firm, Gebr. Trenkle, gem cutters and polishers, also of Freiburg im Breisgau. Mr A. Panzer of Niederrotweil am Kaiserstuhl kindly lent specimens from his private collection. The colour plates illustrating this volume were selected from a much larger number of photographs. They show, firstly, the diverse properties of minerals, and, secondly, the variety of form and colour of individual mineral species.

This volume is not intended to be a text-book, but sets out to convey an impression of the shapes and colours of minerals in their natural form. The minerals selected for particular mention in the text are those illustrated in the plates. For more detailed descriptions of the properties, occurrence and uses of minerals, gemstones and rocks, the reader is referred to the list of text-books and reference works given in the bibliography.

It is difficult to describe in words what it is that makes up the beauty of the crystalline mineral. Apart from the complexity and variety of crystal shapes, which often display near perfect symmetry, the most arresting attributes are probably the great range of colour coupled with the lustre and sheen produced by the reflection of light from the mineral’s surface. The present volume was conceived by the late Dr Ernst Peterson, who wanted to bring before his readers something of the beauty and variety of the mineral kingdom. In Arnold E. Fanck he found a photographer whose genius and infinite care with the camera did full justice to the selecte samples.

The enigma of the crystal shape has always been an intriguing one, and, next to the depth and variation of colour, it is this aspect of minerals which has appealed most to man’s imagination. Among the sagas and folklore of most nations one finds tales of stones and crystals whose supernatural powers could bestow riches or virtues upon their owners. Often, in tribal communities, certain minerals had a religious significance, and many an ancient superstition about a particular stone or crystal has been handed down to the present day.

The many fine gemstones and ore minerals which are to be seen in our museums and in some private collections arouse in the observer a sense of wonder at the intricate crystal shapes and the subtle nuances and variations in colour displayed by many members of the mineral kingdom. Man’s conception of the value of individual minerals has changed many times during the history of mankind. Since Stone Age times there has been an increasing and ever changing demand for the mineral ores from which man has obtained the metals and chemicals which have formed the material bases of succeeding civilisations. Many a war has been fought over the possession of areas which contained important ore-deposits.