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Although of great importance to the chemist, the gemologist is normally not greatly concerned with the chemical composition of gem stones, unless he is interested in their mode of origin, or perhaps in the manufacture of synthetic stones. A knowledge of this subject would be of use if he were interested in the changing of color in certain varieties, although commercially this has been carried out in the past largely on a hit or miss principle. But in general, gem stones are amongst the most stable of substances; they are not easily broken up, and they seldom combine chemically with other matter. They are therefore of more interest to the physicist than to the chemist.
The determination of the individual elements, or combination of elements, which make up a given gem stone would involve the breaking up of the specimen, or even reducing it to a powder, since it would have to be subjected to intense heat and other tests, any of which would render it valueless from a commercial point of view. We therefore depend very little on chemical tests for distinguishing gem stones from one another, but some mention of such tests, which involve the use of a blowpipe, is made in a later chapter. A work on the qualitative and quantitative analysis of minerals should be consulted by the interested reader.
But we can say that the ease with which a stone is fused, if it is fusible, would afford a means of ascertaining its composition. Most gem stones, however, do not respond to this test since they are infusible, or fuse only with great difficulty. Thus diamond, although combustible, is infusible. Ruby and sapphire fuse with the aid of a flux, such as borax, but with difficulty.

Again, there are some minerals which have the same chemical composition and yet crystallize in forms belonging to two different systems. Such minerals are said to be dimorphous. Other substances may form crystals of the same system and yet their angular elements may differ slightly. Alternatively, there are some chemical elements or compounds which will replace each other without altering the crystallographic characters of the compound in which the change takes place. Such substances are termed iso-morphous, and this property of isomorphism is possessed by some gem stones of importance, such as garnet.

The various types of crystals have been arranged in thirty-two possible groups, according to their symmetry, and these groups have been further classified into six systems, according to the relative length and inclination of the crystallographic axes. These six systems may be briefly described as follows:

1. Cubic System  (sometimes called Isometric, Monometric, Tes-seral, or Regular System).
Three equal and interchangeable axes at right angles to each other. Typical forms are the cube (fluor is a good example of a stone usually occurring in this form), the octahedron, e.g. diamond and spinel, and the dodecahedron, e.g. garnet. There are nine planes of symmetry in crystals of this system.
2.    Tetragonal System (sometimes called the Dimetric, Quadratic,
or Pyramidal System).
Three axes at right angles. The vertical axis is longer or shorter than the other two equal lateral axes. A typical form is the four sided prism, terminated by four triangular faces, e.g. zircon. There are five planes of symmetry.
3.    Hexagonal System.
Four axes. The vertical axis is longer or shorter than the other three, to which it is inclined at right angles. The other three lateral axes are equal, and intersect at an angle of 60 ° with each other. A typical form is a six-sided prism, terminated by a single face, e.g. emerald. Quartz, beryl, and tourmaline are among the minerals which belong to this system, in which there are seven planes of symmetry.
4.    Orthorhombic  System   (sometimes  known as  the  Rhombic,
Trimetric, and Prismatic System).
Three axes, which are all unequal but at right angles to each other. Crystals are usually prismatic in shape, e.g. topaz, peridot, and chrysoberyl. There are three planes of symmetry in this system.
5.    Monoclinic System (sometimes called the Oblique, Monosym-
metric, and Clino-rhombic System).
Three axes, two at right angles and the third inclined, all un equal in length. One plane of symmetry only.
6.    Triclinic System   (also known as the Anorthic, Doubly Ob
lique, and Asymetric System).
Three axes, all unequal, and all inclined to each other at different angles. There is no symmetrical plane.
Crystal forms may be divided into pyramid, prism, dome, and pinacoid types. For the sake of brevity, we will tabulate them thus:

(a)    Pyramid Forms. Each face composing this form cuts three crystallographic axes, or would cut them if both faces and axes were produced.
(b)    Prism Forms. Each face cuts the two lateral crystallographic faces and is parallel to the vertical axis. It is thus a vertical form.
(c)    Dome Forms. Each face cuts the vertical axis and one of the lateral axes, and is parallel to the other lateral axis.
(d)    Pinacoid Forms. Each face cuts one axis and is parallel to the other two.
It is somewhat difficult to visualize the axes and planes of symmetry of solid forms, and involved diagrams would probably not make this any clearer to the reader. Much more can be gained by inspecting natural crystals of a perfect form, or by making such forms from wood. The latter are easily obtained should the reader be interested in the study of crystallography, a science which not only has a great bearing on the proper understanding of gem stones but also on the structure and properties of all matter.

The forms of crystals are quite independent of the size of their faces and edges. It is the inclination of their faces that matters, and their faces are always referred to the simplest symmetrical solid to which they are parallel. This parallelism is determined by the measurement of one face to another, and rough minerals in crystal forms may be distinguished by the measurement of these faces alone. The instrument used for this purpose is the goniometer, some description of which is given in a later chapter.

Twinned crystals, or macles as they are sometimes called, may be composed of two crystals, or similar portions of two crystals joined together in such a manner that one would come into the position of the other by revolving it through two right angles round an axis which is perpendicular to a plane. This plane is, or may be, a face of either crystal. Re-entrant angles are characteristic of twinned crystals, and twinning occurs only on a plane parallel to, or on, a crystalline plane.

When two or more crystals appear to have grown together in such a nanner that their corresponding axes are parallel, we have what is known as parallel grouping, and not twinning. Both axes are symmetrically disposed, and the twins can be brought into a state of parallelism by rotating one of them through an angle of 1800 on a twin axis. Repeated twinning, or lamellar or polysyn-thetic twinning, causes false cleavage, e.g. in ruby, which is quite distinct from natural cleavage. Secondary twinning may be produced in a crystal subsequent to its original formation, pressure often being the cause.

For instance, ruby is often composed of very thin plates, which are really twin individuals. It should be noted that such a structure seldom occurs in an artificially made stone or material.
Pseudomorphous crystals are those which present the form of a mineral differing from that of which they are composed. They may be produced by the decomposition of the crystal after it has been formed, or by another substance being deposited upon it so as to assume its form. Sometimes after another substance has been so deposited on a crystal, the crystal may have been removed, and a third mineral deposited in its cast. Thus we sometimes have kyanite replacing crystals of andalusite, malachite replacing cuprite, fluor replacing calcite, and chalcedony replacing fluor.

As we have already stated, the crystal system to which a stone belongs is of considerable importance, for on this many of its physical properties depend. For instance, all similar and parallel directions in a crystal have like properties. It is therefore necessary to be able to recognize some of the most important common forms. A glance at any diagrams of mineral crystals will be of assistance; these show simple forms, but quite often natural stones are found in more complicated forms.

The surfaces of a crystal are called planes, or faces, and a form is a collection of similar faces. These faces are arranged by natuie according to certain laws of symmetry, and it is on this basis that certain groups and systems of crystals have been classified.
In order to describe the form of a crystal, it is necessary to have certain fixed lines of reference (axes) from which can be measured the distance and inclination of the various faces. We therefore imagine lines of indefinite length running through the ideal crystal in certain definite directions and intersecting in the center of it at a point called the origin. In order to fix a plane in space, three lines of reference are required, so that there must be at least three crystallographic axes, and in one case there are four.

The relation of these axes to each other in length and inclination varies according to the class to which the crystal belongs. Thus, these axes may be equal in length, or they may be unequal; they may intersect at right angles or at oblique angles. An axis of symmetry is said to be present when, after turning a crystal through 1800 or 90 ° or 60 ° or 30 °, the crystal looks exactly the same. A plane of symmetry can be defined as an imaginary plane lying between an object and its mirror image; everything on one side has its counterpart on the other. A center of symmetry provides that any face on one side has a corresponding parallel face on the other. A perfect cube will thus have nine planes, thirteen axes, and a center of symmetry.

The term “crystal” has already been used, but it is necessary to understand clearly what is meant in mineralogy by this word. A crystal is a naturally produced solid of a definite form, bounded by smooth surfaces, and possessing definite internal structure. Its external form is really an expression of its internal form as it is built up of an innumerable number of minute particles arranged with geometrical precision. Here we see the chief difference between an artificially produced substance, such as glass, and a substance produced by nature, such as any gem stone. The former will have no definite internal structure, it has not developed from a smaller to a larger like form, and its internal particles are arranged irregularly and indefinitely with no connection between each other.
A crystal grows from a single nucleus, and in order that a large, perfect crystal may be formed, conditions must be such that the molecules find few centers on which to grow. They must develop very slowly and without disturbance so that each molecule has time to settle into its correct place, and the molecules must have enough room to permit of this adjustment. When these condi tions are fulfilled in part only, we get a mass of minute crystals in disarray; the material is then known as being crypto-crystalline, an example being chalcedony. If the resulting mass is of totally irregular structure, an amorphous product is produced, e.g. glass. Regularity of structure and composition throughout its mass is the keynote of the perfect crystal.

This development of crystals in certain definite shapes, or their “habit” as it is often called, is a characteristic feature of most minerals. Some, however, are composed of such small crystaline grains that they cannot be distinguished except under a micro scope. The structure is then said to be compact, and the mineral is described as massive. Jasper, lapis lazuli, and serpentine arc-such examples.
Other minerals are composed of elongated crystals, and when they are very fine, the structure is said to be fibrous. A parallel fibrous mineral is crocidolite, and a silky luster usually accompanies this property. Hematite is often found in nodules of kidney shape, known as reniform, the structure being radially fibrous.

When spheroidal surfaces are displayed, as in malachite, the structure is said to be mammillary. An almond-like (amygda-loidal) form is common with agates, this being caused by the mineral filling a rock cavity of the same shape. There are many other forms, such as capillary, granular, lamellar, and tabular, these terms being self-explanatory. It is interesting to note that stones occurring in the form of crystals, as do most gem stones, are found in this same definite form, no matter where they occur. For instance, the common “habit” of an emerald is an hexagonal prism, and this holds good for emeralds found in the Ural Mountains or in South America. Also the angles between the faces of the crystals of the same species are always the same, wherever the crystal may be found.

Such laces may not be apparent, however, for they are often broken by external agencies, or they may be distorted by irregular development.

Certain different stones are frequently found in the same area, although they may be of totally different chemical composition. For instance, garnets and olivines are often found in the diamond-hearing earths of South Africa, and rubies and spinels commonly occur together in Burma. The experienced geologist, or miner, is often guided by the discovery of one mineral to the presence of smother. The localities where individual gem stones may be found are noted where the varieties are described in detail. Here, we will pursue a little farther the crystalline characters of stones, for on such characters depend a number of their most important properties, such as hardness, cleavage, optical and electrical qualities, and these properties cannot be understood without some knowledge of crystallography, that vast science which deals with the classification of crystals and their mathematical and other properties. The foundation of this important subject was the result of the work of Haiiy (1784) and Sohnche (1867), French and German mineralogists.

Within the earth, there are also natural processes going on which affect rocks. Continuous lateral pressure may cause an interruption in the strata, and fine, flat particles may be induced to re-arrange themselves. Slipping may result, and faults appear in the strata. Twisting, and up and down movements cause other fractures or weaknesses, known as joints. One need not go far to see such examples in our own districts.

As for the majority of gem stones, these are found in the earth’s crust where they were formed, or in secondary deposits to which they have been carried by natural agencies, often some distances away from the original place of formation. In the first instance, they are often a constituent of the rock itself, being formed at the same time as the other component minerals. Sometimes they are attached to walls or cavities in the rock, and in these instances they are not completely embedded. These cavities, known as “druses,” are of a more recent date than the parent rock, and the mineral lining the cavity has usually assumed a definite shape in its formation, that is, it appears as a crystal or mass of crystals. Only one end of the crystal is attached to the parent rock. But when discovered by the miner, these crystals are often found to be broken, or irregular in shape, on account of disturbed conditions. Amethyst is an example of a stone often found in druses, while peridot is an example of a gem stone which has formed at the same time as the surrounding minerals.

But a large number of gem stones are found in secondary deposits, especially in so called gem gravels. These may be in river beds, or former river beds. A variety of stones are found together, and the gravel pits of Ceylon may be cited as a good example where a number of different gem stones may be found associated with other minerals. In such instances, deep mining is not necessary since the gems are found near the earth’s surface.

Organic residues

Organic residues are predominantly composed of the remains of animals or plants. They may be carbonaceous, like coal and peat, siliceous, calcareous, or phosphatic. Chemical residues result, in the main, from the precipitation of substances in solution. They may be found in areas where springs and rivers containing certain salts are subject to conditions of temperature which facilitate the deposition of the solids from solution. Dolomite, a mixed deposit of lime and magnesia carbonates, is an example.

Yet all rocks are constantly undergoing change and destruction. Their component minerals become altered and broken, and their shape is also altered by various natural agencies. Small particles are subject to the process of denudation, and these are often found in sea beds as pebbles, oval or round in form, e.g. agates. Hence we get the term “waterworn,” which is applied to many gem stones
which have been subjected to the action of water and the constant friction with other hard minerals. Crystal faces may be worn very smooth and their original shape quite altered. Weather is an important factor in this work. Frost, wind, and snow split up rocks, and carry the small fragments to lower levels, while rain, which contains carbonic acid, acts as a solvent.

Beneath the surface of the earth, spring water carries on the same work. Rain, gathering into streams and rivers, becomes a transporting agent, and hard rocks are carried in a downward path. If they can withstand their journey, they become “water-worn,” their shape being rounded and their surfaces smoothed. But if they are soft, they may be reduced to small grains. Rocks and minerals, many not so hard as gem stones, undergo this process of transportation and attrition, and such pebbles are often found far away from their original deposits. Pebbles of the native metals are usually known as nuggets, and the heaviest, such as gold, platinum, and cassiterite are very sparsely distributed in rocks.

Foliated rocks

Sometimes we find foliated rocks, which are crystalline in texture but the crystals are arranged in layers. They are inter-banded together and very complex in structure. Gneiss is an example. This rock is much the same as granite, except that the minerals are foliated. Schists are finely foliated rocks, examples being mica schists, hornblende schists and talc schists.
Eruptive rocks may be either intrusive or extrusive. The former is formed by magma solidifying before the earth’s surface has been reached. Dike and plutonic rocks are of this class, and the diamond-bearing kimberlite pipes of South Africa are a good example. These rocks often occur in large masses; they have cooled slowly at great depths and have thus formed crystals. Extrusive rocks are formed by the magma being discharged from the earth’s surface through volcanic vents. Fragmental volcanic rocks  (tuffs) and lavas fall into this group.

Sedimentary rocks are recognizable by the arrangement of layers which they contain. These are called beds, or strata, or laminae if they are very thin. The coarser sediments are made up of pieces of pre-existing rocks which have been able to resist natural weathering and disintegration. The smaller grains are fragments of single minerals, either crystalline, such as quartz and garnet, or secondary products formed by chemical action on primary silicates, such as serpentine. Sedimentary rocks are generally further divided into three main classes; rock residues (fragmental or clastic), organic residues, and residues from solution  (chemical).

Rock residues consist of fragments of igneous or sedimentary rocks which existed before they began to form, as well as primary grains of their more resistant minerals and the secondary products of chemical weathering. When compacted or cemented together, they may be known as conglomerates, breccias, sandstones, shales, or clays. They may be stratified, laminated, or fossiliferous, and usually they are composed of layers or sheets resting on one another.

Now all rocks are aggregates of minerals which, in turn, are simple chemical elements or, as is more general, chemical compounds. As some 60 per cent of the earth’s crust is composed of silica, it is natural that a large number of various silicates are found. In fact, the large group known as igneous rocks are aggregates of such silicate minerals, the amount of silica present varying between 80 and 30 per cent. The proportion of silica determines whether a rock is “acid” or “basic.” In an acid rock, the silica present is in excess of chemical requirements, so what is left over crystallizes as free quartz.

The study of rocks and minerals is a huge subject, and we are really only concerned here with a select number having unique qualities which render them suitable and desirable for personal ornament. From the wide subjects of geology and mineralogy we will briefly touch upon a few terms which are commonly used in the description of gem stones. Rocks, the parent of most jewel stones, have been variously classified at different times, and a large number of names have been used to describe certain groups. But we can say that all rocks may be divided in a general way into two large groups, those of eruptive origin, and those of sedimentary origin.

Eruptive rocks are formed by the cooling of a hot siliceous liquid from a temperature generally above 5000 C. At the time of their formation, they were in a plastic state, this being caused cither by fusion or the influence of heat in the presence of water. We assume that at depths below the earth’s surface, some sixty miles or more, existing rocks are heated to a high temperature. These are prevented from fusing by the overlaying strata, but on account of the presence of water, a partial solution is formed. From this seething cauldron, a plastic mass results, and it is from this magma that minerals crystallize when they have the opportunity. Thus we find crystalline rocks where they have grown, and often we find perfectly formed crystals of one or many minerals. Diamond is a rare example of a single element crystal.

Atoms of mineral

After persisting for many centuries, chemical analysis disproved these beliefs, but during the Middle Ages a great mass of superstition and false ideas were introduced into Europe, mainly from the East. This naturally retarded the progress of science for many centuries.
Yet the atomic theory is not so new as is generally supposed. The idea that all things are composed of minute individual particles, or atoms, seems to have originated in India long before the days of the well known Greek philosophers. Kanada, the founder of the Nyaya system of Hindu philosophy, taught that all material substances exist first as atoms and afterwards as aggregates of atoms. To-day, our ideas about matter have become more comprehensive, but there is still much that requires further research and explanation.
We now regard the molecule as the smallest portion of a substance which is capable of separate existence. Molecules are further sub-divided into atoms, which may be defined as the smallest particle of matter which can take part in a chemical change. But an atom is no longer regarded as an indivisible particle of matter.

According to the present electron theory, now generally accepted, an atom in its normal state consists of an extremely small nucleus to which is attached a positive charge of electricity, surrounded by a number of smaller particles which are negatively charged. The nucleus is termed a proton, and the mass of the atom is concentrated almost entirely in this core. The surrounding particles are negatively charged electrons; these latter revolve round the nucleus at varying distances in closed orbits like a miniature solar system, and the atom is thus an exceedingly empty and open structure.

In addition, there are elementary particles of matter without any charge and of a mass slightly less than that of protons. These particles are called neutrons. There are also positrons, which have a positive charge and the same mass as electrons. The mass of the proton is about 1850 times that of the electron, although in matter the charges of the electron and proton are normally of equal value, but different in sign. The presence of neutrons accounts for the varieties of the same element occurring in forms which differ in their atomic weights. Such elements are called isotopes and their number is considerable, e.g. silica, tin, magnesium, and calcium. The behavior of atoms under certain conditions is the concern of the physicist, and our present idea of the nature and the structure of the atom is based on the work of Thompson, Rutherford, and Bohr. Any discovery concerning the composition and behavior of matter in general is naturally of great importance to those who study gem stones. The properties of stones are fundamentally the result of the arrangements of the included groups of atoms, together with their nuclei and electrons, in a normal state.

In the normal state of an atom, the negative charges just balance the central positive charge, but disturbance causes chemical and physical changes in the process of a re-adjustment. Such changes occur with all substances, especially when subjected to tremendous heat and pressure, such as deeply buried elements in the earth. In the course of such changes, the combination of a number of elements may eventually result in what we call a gem stone.

Thus below the upper soil of the ground lie various kinds of rocks, which differ vastly in composition. These rocks are made up of many minerals, each of which has an almost definite chemical composition, and through the course of some hundreds of thousands of years, Nature has gradually built up these rocks, particle by particle. For although changes go on unceasingly below as well as above us, these changes are, for the most part, very gradual.

Rocks are being constantly subjected to various strains and pressures, and it is a well known fact that the deeper one descends, the hotter does it become; in fact, the temperature increases one degree Fahrenheit for every 60 feet of descent, so it will be seen that heat is an important factor in the formation of new rocks.