Ennobling Rarity

The subject of the rarity of gemstones, which so much enhances their prestige and value, leads us down into the dark, inaccessible depths of the earth's interior—to the birthplaces of the gemstones—where they were formed under the exertion of enormous natural forces. It was primarily there that the conditions critical for beauty, rarity, and durability were fulfilled; but it is strange and surprising that the majority of these most beautiful and valuable creations, conceived in the earth's womb, are composed of the most ordinary materials; namely, carbon (C), alumina (aluminium oxide, A1203), silica (silicon dioxide, Si02), lime (calcium carbonate, CaCO,), and others besides.

Considering the worldwide distribution of these common substances, it is remarkable that gemstones are so rare. Apart from the requirement that, in the first place, factors promoting crystallization must be present, in many gemstones of simple composition it is the extremely rare coloring agent (e.g., chromium in alexandrite, ruby, and emerald) that determines the distinct rarity; in gemstones of complex composition, on the other hand, rarity is occasioned by an important but minimally represented constituent such as fluorine in topaz and boron in tourmaline. Many a mineral formation is virtually unique, as in the case of benitoite, because the participating chemical elements were present only once, and in a single locality of the earth, at the same time so as to combine in the correct stoichiometric proportions for the mineral concerned wholesale diamond rings.

By far the majority of gemstones have grown as crystals. This means that they consist of ordered matter, in which the structural elements—that is, the atoms—are arranged according to rigorous laws. Of two adjacent atoms, one is always positively, the other negatively charged. These oppositely charged atoms, named ions, attract each other. The force holding them together is called cohesion and determines the solidity of the crystal. Under favorable conditions in melts, solutions, and gases, ions which are oppositely charged and complementary to one another may be found together; they form up in lines, construct planes, and build themselves into space-filling crystal lattices designer wedding bands, arranged in accordance with definite laws, their inner structure being revealed on the outside by an architecture of many faces.

In connection with the rarity of gemstones, the question now arises: under what conditions of mineral formation can specially beautiful and large crystals originate? Geology teaches us that the great developmental processes which the crust of our earth has undergone in the course of its history have led to the formation of mineral deposits under extremely varied conditions. They are governed by a twofold event, whose phases are closely related to one another: the formation of mountains and the uprise of molten masses, so-called magmas (from the Greek magis, kneaded masses), from the interior of the earth. An extensive zone of such molten matter, which on account of its chemical composition (silica and magnesium) is designated sima, spreads beneath the crust of the earth engagement ring settings. The molten masses of the sima in the depths of the earth are inaccessible to us. Incontrovertible proof of its presence is provided by observations of the propagation of earthquake waves and naturally by the basaltic lava flows of volcanoes. From this subterranean sima diamond originates as an early crystallization and reaches the earth's surface by way of volcanic

chimneys which break through it. Of similar origin is pyrope. Some of the Siam and Australian sapphires and many zircons also occur in basaltic lava flows from the sima.

The spherical shell of the sima is enclosed on the outside by the solid crust of rocks, the lithosphere. Into the latter, melts from the region of magmatic rocks are forced during mountain building; their composition is characterized by a combination of silica and a/umina, whence the material is named sial. These sialic melts bring us a much greater number of gemstones in their cooling products—the rocks and mineral deposits of the earth's surface resulting from solidification through cooling.

The congealing of a magma into rock through crystallization is a very complicated process which has taken place over millions of years in three great cycles, namely, the magmatic, the metamorphic, and the sedimentary cycles. Within the magmatic cycle the sialic magmas have, during their crystallization, gone through a series of stages which are subdivided according to their sequence and cooling temperatures into the liquid-magmatic (i 50o-7oo°C), pegmatitic (7oo-50o°C), pneumatolytic (5oo-40o°C), and hydrother-mal (400-1 oo °C.) phases. In the liquid-magmatic phase the main crystallization took place in the molten zone while the definitive formation of the plutonic rocks was going on; examples of such rocks are granites, diorites, and gabbros. Thereby, according to the composition of the magma, the most important rock-forming minerals, such as mica, feldspar, quartz, olivine, and others, were segregated out in orderly succession. The gemstone labradorite, with its variety spectrolite, as well as peridot and some zircons are likewise rock-forming minerals of this phase yellow diamond rings. The medley of accessory gemstones crystallizing out, such as apatite, beryl, spinel, tourmaline, and others, were still at this stage unimportant owing to their microscopically small size.

As the crystallization process of the liquid-magmatic phase proceeded the magmatic melts became more and more exhausted, leaving behind residual melts, in which the so-called volatile constituents of the magma, such as water, boron, chlorine, fluorine, carbonic acid, phosphorus, sulphur, uranium, zirconium, and so on, then precious and heavy metals, as well as a whole series of rare elements, were meanwhile concentrated by differentiation. Here, too, silica and alumina still played a decisive role. The internal pressure of these superheated melt solutions was very high, and the volatile residual substances gushing copiously out of the cooling magmas raised it even higher. Thus these residual melts emerged from the realm of the plutonic rocks under high pressure and forced their way along cracks and fissures—so-called gangues—into the adjacent country rocks, where their chemical and thermal action produced great changes.

to pegmatites. The pegmatitic minerals crystallizing out from hot, watery residual fluids found specially favorable growth conditions. The content of volatile constituents made these pegmatitic melts not only very fluid and mobile so that large crystals could grow in them, but also chemically very active, too. Despite the paucity of nucleation sites they were astonishingly eager to form crystals; for the chemically active mixtures stimulated crystal growth, as so-called mineralizers. Thus the crystals attained the requisite purity and size for gemstones. The widespread fairly large druses in many pegmatite districts are well-known as storehouses of exceptionally beautiful crystals. For the zone of pegmatite formation is also the home of the largest crystals known in the solid crust of the earth, and thus at the same time the most important nursery of gemstones; for example, it produces apatite, beryl, chrysoberyl, euclase, fluorite, kunzite, moonstone, sapphire, topaz, tourma­line (black), and again some of the zircons, and many other somewhat less significant gemstones.

As a result of further cooling of the postmagmatic residual melts the volatile constituents became even more strongly enriched, and there arose gaseous and partic­ularly hydrous solutions, from which developed the pneumatolytic deposits (Greek pneuma = breath; French lyein = to loose). The extremely reaction-prone pneumatolytic mineral solutions were able to force their way, even more strongly than the pegmatitic ones, into limestones, dolomites, and clays and to act upon them, as they are very easily chemically attacked. Through the strong chemical activity of the volatile substances, a brisk exchange of materials took place, which led to alteration of the adjacent country rocks, so-called contact metamorphism, from which quite new minerals and rocks—con­tact minerals and contact rocks—were produced. In this manner deposits were formed, which have become world renowned for their amazingly varied mineral associations (parageneses, from the Greek para = near, beside, and genesis = origin) and beautifully developed crystals, for example, those of Ceylon, of Mogok in Burma, and in the Urals.

Offspring of the pneumatolytic phase are ruby, sapphire, and spinel in contact metamorphosed limestone, with zircon as their companion (Burma and, in part, Ceylon); emerald in metamorphic schists (Rhodesia, Transvaal), and, together with alexandrite, in contact metamorphosed biotite schists (Urals), lapis lazuli in contact metamorphosed limestones (Afghanistan and Chile), and so on.

The hydrothermal phase (Greek hydor — water, thermos = hot) formed the close of the magmatic cycle; in its course hot, watery mineral solutions rose up from the depths into clefts and porous zones of the rocks and penetrated, according to the available space, into fine veins or massive lodes. From these hydrothermal solutions, which carried mainly silica, as well as residual constituents of the rock-forming chemical elements and heavy metals, the dissolved substances crystallized out through cooling and formed vein fillings. To these, in addition to fluorite, belong chiefly the varieties of the quartz group: amethyst, rock crystal, smoky quartz, and citrine, as well as precious tourmaline and also, in part,

topaz. Of hydrothermal origin, too, is the emerald found in calcareous shales at Muzo, Colombia. Hydrothermal mineral parageneses are the most important precipitators of gold, silver, copper, tin, zinc, uranium, and many other heavy metals, and are on that account of special economic value. Between the individual phases of the magmatic cycle additional multiple transitions are known.

A further important rock-forming process was the metamorphic cycle, which included the large group of the crystalline schists. They originated from already existing magmatic or sedimentary rocks which, bedded in the earth's crust, did not remain unaltered, but, under the influence of increased pressure, and access of heat and circulating solutions, suffered a recrystallization (metamorphism). These metamorphic processes were

mostly associated with the uprise of magmatic melts or with mountain-building forces, whose often unilateral pressure imparted a slatelike structure to the resultant rocks. Among the metamorphic rocks belong such well-known ones as the gneisses, mica schists, serpentines, and marbles. Among the gemstones, they offer us only almandine— frequently found as an accessory mineral in gneisses and mica schists—as well as both the jade minerals, jadeite and nephrite. Certainly in the last two, additional contact metamorphism of sialic magmas has also been in play.

The cycle of weathering processes, which in itself contributes only unattractive rocks and minerals, is poor in gemstones. There are only four gems which are weathering products, namely, turquoise, chrysoprase, malachite, and azurite. Turquoise is a by­product of the weathering of acid magmatic rocks in conjunction with the deposition of copper ores nearby. Its formation derives from circulating meteoric waters and is promoted by the continuing attrition of the rock by hot water solutions. Thereby alumina was released from the feldspar of the magmatic rock, while phosphoric acid was set free from the apatite. The color-active copper originated from the copper ores which were interbedded in the rock. Chrysoprase is the end-product of a complicated chain of unique altering and concentrating processes, in which nickel-bearing basic rocks were involved. At the end of these processes chrysoprase developed in lodes and cavities of the weathered masses as a new deposit in the form of a finely fibrous quartz, between whose fibers nickel grains were embedded, giving it its apple-green color. Green malachite and blue azurite resulted from surface weathering of copper ores under the action of water and carbonic acid.

The occurrences emerging from all the processes of formation described above and responsible for the rarity of gemstones constitute the actual birthplaces, that is to say, the primary deposits of gemstones.

In the weathered debris which accumulated from the erosion of rocks, gemstones —resistant to attack by weathering factors—lie well preserved, and can be recovered easily by washing. This kind of deposit is called "eluvial drift," and gemstones are often strongly concentrated in it.

(higher specific gravity) gemstones sank and collected in hollows, or so-called pockets. As this accumulation took place by water, and likewise a natural puddling occurred, these secondary occurrences are often called placer deposits or alluvial gem gravels. Such gravels are found both in present-day stream beds and valley bottoms as well as in the older river terraces and bank dunes deposited higher up. In this way Nature gives them to us in the truest sense as a gift; for through enrichment in the detritus, a natural selection of the best qualities has occurred, and, moreover, mining can be operated at considerably reduced cost.

In the world map of gemstone deposits one is struck by their restriction to, or rather, their concentration in the zones near the equator. Obviously this can only signify that formations of the gem-rich phases of the sial cropped out particularly abundantly in these districts. In the course of world history the intensity and dimensions of sialic magma uprisings in conjunction with mountain-building have consistently decreased. Our young fold mountains—the Alps, the Andes, the Himalayas—which rose in the Tertiary era only—contain far fewer magmatic rocks and postmagmatic derivates, e.g. pegmatites, than the old mountain stumps of the former massive primeval continents of the earth. Of these, the hypothetical continent Gondwanaland corresponds with the regions most prolific in gems, for remnants of it form the basement of South America, Africa, Madagascar, Southeast Asia, Borneo and Australia, where valuable gemstones are chiefly found. North America lies on a continental shield which geologists have named Laurentia. By far the greater northern part of it, however, is overlain by deposits of the Ice Age, so that this subsurface is not accessible to us.