The nature of the diamond had puzzled scientists for hundreds of years, and it was not until nearly the end of the seventeenth century that its relationship to carbon was even suspected.
The first well-documented experiment on a diamond was carried out by two Italian academicians in the presence of the Grand Duke Cosimo III in Florence in 1694. They set up a large burning glass, focused the beam on a small diamond and saw it "crack, coruscate and finally disappear," leaving a minute quantity of blue ash. But it was the French physicist Babinet, along with eminent colleagues like Lavoisier, who proved beyond reasonable doubt that a diamond was carbon in exceptionally pure form. This apparent paradox caused Babinet to observe:
And what is a diamond? The most precious thing in the whole world. And what is carbon? The most common material that is known. It not only exists in vast quantities in the bowels of the earth, but plants and trees of every kind contain it in an inconceivable quantity. And yet the diamond and carbon are identical. Diamond is crystallized carbon.
One of Lavoisier's most celebrated experiments was to place a diamond in a bell jar filled with oxygen which rested in a basin containing mercury. The rays of the sun were then focused on the diamond by means of a large burning glass. After the diamond had been consumed, the bell jar was found to contain great quantities of carbonic acid, indicating to Lavoisier that the diamond was composed principally of carbon. Later experiments by Humphry Davy in England prompted him to conclude that the diamond was composed of carbon and nothing else, a view that was first contradicted in 1841 by Dumas and Strass, whose numerous experiments revealed minute traces of other elements, notably nitrogen and aluminum. It is these slight impurities that deform the perfect crystalline structure of the ideal diamond, and can have a marked effect on the physical properties of a stone. This knowledge is now being used in the production of synthetic diamonds to "tailor" them for particular applications in industry (see "Harder than Steel").
It would be an exaggeration to say that not much more is known about the origin of diamonds today than two thousand years ago when they were believed to be splinters of stars. Nevertheless, it is true that there is still no unanimity among geologists about exactly how and where diamond is formed. As late as the nineteenth century, theories about the origin of diamonds had been shaped by the fact that the stones had always been found on or very close to the surface, either in riverbeds or in the beds of rivers that had dried up ages ago. And given the unique qualities of the diamond together with its rarity, it is perhaps not surprising that the "heaven-borne" theory should have been the most enduring. Even as late as 1869 the Gentleman's Magazine of London reported that a "Continental experimentalist" had declared that the intense cold of stellar space disassociated and crystallized carbon from "masses of meteoric nature coursing through space" and caused diamonds to fall from the sky. The editor went on to comment that "the location of diamonds upon the earth agrees much better with the hypothesis of a sky-source than an earth-source," and that "those Cape specimens now attracting so much attention are found on the surface of the ground only; it is of no use to dig for them." Still, the "Continental experimentalist" may well have a point. Diamonds have been found in meteor craters at Novo Urei in southeastern Russia and at Canyon Diablo in Arizona, although most scientists believe they were created by the heat and pressure of impact and not carried to earth in the meteors.
It was not until the discovery of the "dry diggings" at Kimberley in 1870, coupled with the determination of the miners to excavate every inch of their dearly bought claims, that it became clear that diamonds came from below and not from above. It was also clear that diamond was invariably associated with one particular type of rock, and that this rock was only to be found in clearly delineated areas. Since the rock—and the diamonds—persisted at depth, it was soon suggested that these "pipes" were volcanic in nature and that diamonds had been formed out of carbon under intense heat and pressure deep in the bowels of the earth. They had then been forced toward the surface when those long-extinct volcanoes had erupted millions of years ago. This strange diamond-bearing rock—soon to be called kimberlitic was assumed to be nothing more than solidified lava. But as mining progressed at Kimberley, it was discovered that the pipes were not great volcanic funnels plunging into the earth's core. Rather, they were like irregularly shaped carrots, decreasing in area and petering out in a number of fissures at varying depths from a few hundred to several thousands of feet. Furthermore, relatively few of them contained any diamonds at all.
A great deal has since been learned from the making of synthetic diamonds, and perhaps the most widely accepted current theory is that since diamond forms at pressures and temperatures between 0.5 million pounds per square inch at 700°C and 1.3 million pounds at 2,500°C, the formation must have taken place at depths of at least 120 miles. Chemical studies pointed to the ultra-basic rock peridotite in its molten form as the most likely to have provided the right conditions for the creation of diamond from carbon. The process of crystallization is assumed to have been long and slow, and the theory goes that conditions remained stable for a considerable period as a result of the pressure of carbon dioxide gas below the peridotite layer being equal to the pressure from the earth above. When the pressure below became too great, the balance was changed and the diamond-bearing magma (molten rock) was driven explosively toward the surface. On the way it picked up other rocks and minerals, forming itself into the "geological plum pudding" that we now call kimberlitic, eventually erupting through the surface of the earth and solidifying.
The violence of the eruption toward the surface is believed to account for the fact that so many diamonds are chipped or cleaved. In the De Beers Mine, two large and unusually shaped pieces of diamond called cleavages were found simultaneously in different parts of the mine, and yet fitted together exactly. Even the huge Cullinan is a cleavage, and there is still a hope that one day the Premier Mine will disgorge the other half of the biggest diamond ever found. Other curious items have been found in kimberlitic pipes including pieces of fossilized wood, ancient ostrich eggs, and even a headless human skeleton with the skull discovered fifty feet away at half the depth.
Another puzzling phenomenon is the diminishing yield and average size of diamonds as a mine gets deeper. In the Premier mine, for example, the blue ground down to the 400-foot (125-meter) level yielded between 0.80 and 1.29 carats a load (1,600 pounds), whereas the next 100 feet (33 meters) yielded barely 0.19 carats. The natural assumption from this fact is that far more diamonds have been dispersed across the surface of the earth by erosion of the top of the pipes than have been extracted from, or remain in, the pipes. The South African pipes are estimated to be 120 million years old, and geologists believe that since then a slice of earth (and kimberlitic) between half a mile and a mile (800 to 1,500 meters) has been eroded from the Kimberley region. If this is the case then it is calculated that some 3 billion carats of diamonds have been "lost" over the ages.
This dispersal theory could go a long way toward explaining another diamond mystery—the remarkably high proportion of fine gem-quality stones among diamonds found in the marine terrace deposits of South West Africa. The fact that only about 3 percent of the stones found there are of industrial quality compared with 20 to 30 percent in the pipes has caused some geologists to suggest that the marine terrace diamonds come from a different source than other alluvials, perhaps even from pipes under the seabed.
There is no reason why diamond-bearing pipes should not exist under the sea. A more obvious explanation, however, would seem to be the merciless pounding the diamonds would have received at the hands of the waves on this storm-swept coast over millions of years. The larger crystals would have settled more rapidly than the smaller ones, many of which would have been swept out to sea, while the irregular, the fractured and the poorer-quality stones would have simply disintegrated. Laboratory tests simulating wear and tear have shown decisively that a good-quality gem stone will survive un-harmed the sort of treatment that can totally destroy a poorer-quality stone.
Probably the best-known quality of the diamond is its exceptional hardness. The mineralogist Mohs devised a scale, now known as the Mohs scale, in which minerals were ranked from 1 to 10 according to their relative hardness, which was determined by a scratch test: a given mineral on the scale will scratch those below it and be scratched by those above it. Diamond comes right at the top of the list, with no other natural mineral even approaching it in hardness. The scale is not a linear one, and the hardness gap between diamond and corundum, the next hardest mineral on the scale, is much wider than the gap between corundum and talc, the softest:
Only a diamond will scratch another diamond. It is this quality of exceptional hardness that makes it so valued as an industrial material used, for example, in the heads of rock drills and in the tips of cutting tools.
But although diamond is the hardest substance known, its hardness is directional because, like wood, it has a grain. This was first discovered by the early diamond cutters, who looked upon the knowledge as one of the great secrets of their trade. Since the variation in hardness may range from ten times in one direction to a hundred times in another, appreciation of the fact was clearly vital to successful cutting. For the same reason the diamond is also relatively brittle and can be shattered by a misplaced blow. The cleaver, for example, may study a large stone for days or even weeks in order to establish the exact direction of the grain before attempting his task.
Once it has been cut and polished, the diamond has a number of unique optical properties which account for its superior ranking to colorful and brilliant gems such as rubies, emeralds and sapphires. An exceptionally high degree of luster and brilliance are the most important of these qualities. Luster refers to the quality of light that is reflected from the surface of a material; it is graded from practically nil on certain materials up to a uniquely high level in the case of the diamond, when it is called "adamantine luster." Brilliance is concerned both with the "life" of a stone (the degree of light that is reflected from the surface and interior of the diamond) and with its "fire" (the amount of refraction and color dispersion that is achieved). The more that light is refracted and split into the colors of the spectrum, the greater the amount of fire a stone is said to have. It is the job of the diamond cutter to achieve a fine balance between the fire and life of a diamond, as maximum fire is not consistent with maximum "life." The early cutters were well aware of these qualities. As the Sir John Mandeville Lapidary observed in 1561, the diamond "seems to take pleasure in assuming in turn the colours proper to other gems."
A further distinguishing feature of many diamonds is that they will glow or fluoresce when exposed to ultraviolet light; they may also phosphoresce, or continue to glow, after the removal of the light source. Every diamond is different both in the color and intensity of its fluorescence and phosphorescence, a fact which can make for positive identification of two apparently identical stones of pieces of jewelry. The Hope diamond, which is blue, fluoresces red, for example; while the Maximilian, which is also blue, fluoresces violet even in daylight.
However, despite this unique catalogue of the optical attributes possessed by the diamond, the fact remains that in the rough state in which it is found, a diamond is often not recognized for what it is. It does not sparkle and flash. On the contrary, it is a rather dull, ordinary-looking pebble whose principal distinguishing feature is its shape. The reason is that many diamond crystals are coated with a thickness of inferior quality and badly formed diamond. The coating may be gray, green, brown or black and is usually found to contain many small inclusions of foreign material. It is not necessarily indicative of the standard of the rest of the stone and a dull, dirty coat can cover a diamond of the highest quality—or one almost as dull and dirty as itself.
Diamonds, like most minerals, are crystal-line with a regular internal structure that is not necessarily reflected in the external form. Growth occurs in layers but it is by no means uniform. Some faces may be underdeveloped and others overdeveloped; the whole crystal may change its orientation during growth; or two or more crystals may grow locked together. As a result the diamond appears in many more forms than is suggested by the popularly depicted eight-sided crystal called an octahedron. It appears in other single crystal shapes as well as in formless crystalline masses.
Of the seven main systems of symmetry into which crystals are divided, diamond falls into the cubic system, the most symmetrical of all. The possible forms of regularly shaped diamond crystal are illustrated on pages 66 and 67 along with their number of faces.
The octahedron is the most common of these crystals of regular shape. Even more common than the octahedron, however, are the pieces of no recognizable form. These may be distorted crystals or pieces that have been broken or worn into irregular shapes. It is these diamonds that provide the real challenge to the cutter. Once he has studied them and found the crystal directions, he can often make a higher recovery than he would on a regular crystal. The huge Cullinan was such a stone.
There are literally thousands of divisions into which rough diamonds could be classified as they come from the mines, but in order to simplify the process they are restricted to four main shapes. In order of value and importance to the gem cutter, they are as follows:
1) Stones unbroken crystals of regular formation.
2) Cleavages broken or irregularly formed pieces.
3) Macles twinned crystals, flat and triangular in form.
4) Flats irregularly shaped pieces with flat parallel sides.
This is the practical working division and, of course, it cuts right across any academic or scientific one based on ideal crystal types. The classifications of diamonds already mentioned refer only to gem crystals, but since 80 percent of production is destined for industrial use, there is a broader grouping that divides diamonds into:
1) Gem diamonds
2) Industrial stones, including:
a) shaped stones
b) whole stones
The difference between gem and industrial diamonds is purely one of quality and color. The imperfections that affect quality and color may take the form of fractures or fissures or of minute inclusions of other minerals that were present in the original magma when the diamond was formed. The most popular shape for the gem cutter is the octahedron. For industrial use, the dodecahedron and other more rounded crystals are generally preferred, although octahedrons are still regarded as more suitable for use as truing diamonds for shaping grinding wheels and for setting in the tips of rock drills. Irregularly shaped stones are usually used as glass cutters' diamonds and for setting in stone saws.
Boart (an early Afrikaans word for "bastard") is a minutely crystallized gray or black diamond mass which is not usable in individual crystals for any industrial application. It is therefore crushed to powder for grinding and polishing purposes. Boart has its own numerous classifications, one of the most interesting of which is ballas or shot boart. This is found in the shape of a ball and with no crystalline faces or edges and no lines of cleavage, it is virtually indestructible. The Brazilian name for boart is carbonado.
It would be a mistake, however, to think that boart or carbon ado is dull and uninteresting. There was a case in 1927 of what was apparently a 33-carat piece of boart being found to contain a small red diamond of exceptional quality at its heart. It eventually produced a 5.05-carat gem when cut.
On rare occasions boart exists in a form that enables it to be cut to create a truly unique gem. The best-known black diamond is the celebrated Black Orloff, a 67.5-carat stone cut from a 195-carat rough of Indian origin. But a better and much more recent example is the beautiful stone known as the Amsterdam. While the Orloff is more of a dark gun-metal color and partly translucent, the Amsterdam is totally black and impervious to light. The stone arrived at the offices of D. Drukker and Sons in Amsterdam in 1972 in a parcel of mine boart destined to be crushed into diamond powder or to be broken up into smaller pieces for other industrial purposes. At the time the 55.85-carat rough would have been valued at no more than $5-6 a carat. Drukkers tried to cleave the stone and immediately became aware both of its exceptional hardness and of the fact that the splinters were not in the least transparent, but of the deepest black. They decided to proceed with cutting and polishing the stone. The result is a pear-shaped 145-facet black diamond weighing 33.74 carats, one of the rarest gems in the world. Both the Amsterdam and the Black Orloff dramatically underline the fact that the diamond is unique in all its many forms.