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Minerals - Mother nature’s miracles

Minerals are only one of the many products of nature which have been divided into three great classes, animal, vegetable, and mineral. Our study of gem stones is almost exclusively concerned with the mineral kingdom. The geologist is concerned with the nature of rocks, the mineralogist deals specially with the minerals of which rocks are composed, while the chemist specializes in the chemical composition of minerals. Gemology is therefore a specialized branch of mineralogy, although a knowledge of the other sciences is necessary for a complete understanding of the nature and properties of gem stones.

Minerals differ from the other two great divisions inasmuch as no animal or vegetable life has taken part in their formation. They are composed of elements; an element being a substance which has not, as yet, been decomposed into a simpler entity, and whose structure is made up of a definite number of atoms or molecules.

Our knowledge of the structure of matter has increased con siderably during the last century, and in late years, many new theories by physicists have been advanced. Before, many theories which we have now proved to be false persisted through many centuries. For instance, Thales (640-550 B.C.), who has been called the first natural philosopher, taught that everything is produced from water, to which it ultimately returns. He thought that amber has a soul because of its power of attracting certain other minerals. Water hardened by long frost was believed to turn into rock crystal. The idea that water might become earth was very generally believed for many centuries.

Empedocles (440 B.C.) asserted the existence of four elements, earth, water, air, and fire, and this theory was current through the Middle Ages up to a couple of centuries or so ago. These four elements were not regarded in their strictly literal sense, but rather as types of classes. For instance, with fire were classed light, heat, and lightning. All liquids were classed with the element water, and all solids with earth. Fire was the soul, and the other three elements the body.

Now away from the human eye and beneath the earth’s surface a wonderful and varied world lies buried. Here, all kinds of rare and precious materials remain hidden, these being but a few of the results of the tremendous natural processes which go on century alter century, uncontrolled and unaided by man.

I( is just by chance that certain solids, liquids, and gases may be present within the earth and in one particular region where great heat and pressure are being applied. This leads, perhaps abruptly, or more often over a long period, to the formation of
new solids, and among these may be the rare and beautiful materials we know as gem stones. For hundreds of thousands of years they may lie buried until a mere accident may bring them to light. But more likely they will remain in the bowels of the earth for further countless ages.

It is interesting to note that history gives us no description of the first discoveries of precious stones by man. The ancient writers, travelers, naturalists, and historians give us no information as to how mines were worked, and this leads us to think that the first jewel stones known to man were found by chance either on Or near the earth’s surface. Various stones such as diamond, lapis lazuli, amethyst, and particularly amber and pearl, were surrounded by legends which tried to explain their formation. Other stories- were no doubt concocted to deter competition or to keep the masses in ignorance.

But mines did exist many years back, evidence of which we have in the re-discovered emerald mines by Caillaud in Egypt. The mountainous country between Abyssinia and Egypt produced some of the earliest known emeralds, but the site of the African mines whence the ancients drew their gem stones is unknown to us.

It will be found, however, that our present conception of matter will not conform to a simple strict definition of a mineral. For instance, it would be difficult to express the composition of tourmaline by a fixed chemical formula, and there are minerals having similar chemical constituents which almost merge into each other and are yet different in appearance and name. Others contain traces of elements which affect their appearance or stability, yet these elements do not appear in any of their generally accepted formulae. What we have stated is a general rule only. Further, there are some materials used in jewelry which do not come within any definition of stones, such as pearls, coral, amber, and ivory, since these are of organic origin.

A generally accepted, although somewhat arbitrary, division exists between what are called “precious” and “semi-precious” stones. The latter term is misleading, but it is still used for want of a better description of those jewel stones other than diamond, ruby, sapphire, and emerald. These four stones are usually considered as being precious, although this term is not based solely on commercial value. There are numerous other stones which are both rare and beautiful, and at the same time reaching high commercial values. Such are, for instance, black opal and alexandrite. Jade necklaces have reached the price of $70,000 (£25,000) in America, while poor sapphires, a so-called precious stone, may cost less than a dollar a carat.

All such other stones are sufficiently attractive for ornamental purposes when they are suitably cut and polished. Many have depth of intrinsic color and are sufficiently hard to withstand ordinary wear, properties which are essential in gem stones. But commercial values vary greatly with fashion, and these stones are more subject to such current influences than are the precious varieties. The term semi-precious is therefore misleading; it does not mean half precious, or half real, and it would be as well if another term came into general use in its place. Among the many beautiful stones of this nature are zircon, chrysoberyl, topaz, spinel, jade, tourmaline, amethyst, and kunzite, all real stones which are seen in mounted jewelry but which, in many instances, are found in too abundant quantities to make them rare and therefore of high value. The essential qualities which make a stone “precious” are generally considered to be beauty, rarity, and durability.

The description of gem stones and their properties necessitates the use of certain terms, which we will now proceed to define. If we do not understand these terms, they will appear to be some meaningless jargon which is being constantly repeated, but actually they are only a clear and concise method of expressing certain physical and chemical properties.

For instance, we may know what a “crystal” means, or what “hardness” generally denotes, but we must be more exact when we use such words in connection with stones. The study of gemology entails the grasp of some kindred sciences, and the wider knowledge we acquire of certain branches of physics and chemistry, and more particularly crystallography, light, and the structure of matter, the more we shall understand gem stones. Yet without going into such subjects deeply, there is plenty of interest for everyone in these precious and beautiful products of nature.

With very few exceptions, all gem stones are natural minerals. They are, therefore, the product of an inorganic process and are composed of the same substance throughout. All have a definite chemical composition, which generally may be expressed by a chemical formula. In other words, all true gem stones are formed naturally, without any artificial assistance, and they are homogeneous in composition. They are usually composed of two or more elements which are chemically united. Diamond, being pure car bon only, is an exception.

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One of the outstanding properties of most gem stones is hardness, for often it is only by this merit that they have survived their long journey from mountain heights to valleys and river beds. In general, gem stones are amongst the hardest of nature’s products, and diamond takes pride of place, being the hardest substance known.
By hardness, we mean the resistance which a stone offers to the separation of its particles. The degree of hardness varies considerably, and with certain exceptions, the same stone will always exhibit the same degree of hardness. This degree can be measured relatively by scratching two minerals together, and hardness points, which consist of a graduated series of different minerals of varying degrees of hardness, are sometimes used. Fragments of these minerals are so set that they can be conveniently applied to the surface of a given specimen. If a mineral definitely scratches another, it is naturally the harder of the two, but this is an elementary test and one on which no great reliance can be placed.
A more exact instrument was devised by Brinell, a Swedish scientist. By using his method, the relative hardness of any substance can be tested, and the Brinell hardness number is now generally recognized, particularly when metals and alloys are being considered.
There are other hardness testers, such as the Rockwell machine and the Vickers diamond pyramid tester, but all are based on the same principle, that is, a diamond point is pressed or revolved on a surface for a given time under a definite load or pressure, and from the nature and area of the indentation or abrasion made the hardness number can be calculated.
A scale of hardness was drawn up by the mineralogist Mohs, and this is still in general use with gem stones, although it only shows relative hardnesses with no degree of equality between the different grades. Diamond is given the hardness of 10, and other stones to denote the lower degrees are
9    Corundum.
8    Topaz.
7    Quartz.
6    Orthoclase.
5    Apatite.
4    Fluor spar.
3 Calcite.  2Gypsum.  1Talc.
The hardness of most gem stones, according to this scale, is given in the Appendix, but it must be clearly understood that the figures in this scale have no quantitative significance. In other words, diamond with a degree of 10 is not just twice as hard as apatite, which is rated at 5. It is, in fact, very many more times as hard, the actual difference in hardness between diamond and corundum (ruby or sapphire) being far greater than that between corundum and talc. Diamond is far and away the hardest of gem stones.
Gypsum, a variety of which is alabaster, is so soft that it may be scratched with a finger nail. Window glass is about 5, a pen-knife will scratch minerals up to about 6 1/2 and the finger nail will mark stones having a hardness of about 2 1/2. Each stone given in the list of hardnesses will be scratched by a stone of higher degree.

Compounds of metals and oxygen are called oxides, and metallic salts of carbonic acid are called carbonates, e.g. Iceland Spar. In like manner, hydro-fluoric acid gives us fluorides, e.g. fluor spar, phosphoric acid, phosphates, e.g. turquoise, silicic acid, silicates, e.g. emerald, sulphuric acid, sulphates, boracic acid, borates, titanic acid, titanates, chromic acid, chromates, and hydrochloric acid gives us chlorides.
But if all gem stones were analyzed, it would be found that the following elements would be the most conspicuous:
Silicon, aluminum, iron, carbon, oxygen, hydrogen, copper, calcium, magnesium, sodium, manganese, chromium, potassium, phosphorus, sulphur, zinc, zirconium, and beryllium. Silicon does not occur in a free state in nature, but usually in the form of silica, which is a combination of silicon and oxygen. In this form, it constitutes a considerable portion of the earth’s crust—some Go per cent—and many well known varieties of gem quartz are composed of this material. It figures largely also in the composi-lion of flint, sand, and all silicates.
With the exception of diamond, which is almost pure carbon, most of the other gem stones are a natural combination of two or more of the above named elements. Thus rock crystal is a combination of oxygen and silicon in certain definite proportions, ruby is a combination of aluminum and oxygen, although most gem stones are more complex in composition than these two examples. It should be noted that the color of a stone is an optical property and is due to a very minute inclusion of some foreign matter, which might be organic or inorganic, and which is rarely expressed in the chemical composition of the stone.