Birds and Nature, Vol. 12 No. 4 [September 1902]

THE DIAMOND

The Diamond is generally conceded to be the most beautiful as it is the most important of precious stones. While other stones at times exceed it in value, weight for weight, in total importance as an article of commerce other gems are hardly to be compared with it. Out of thirteen and one-half millions of dollars’ worth of precious stones imported into the United States in 1900, twelve million dollars’ worth were Diamonds. Not all this amount was employed for jewelry, since there is a large utilization of the stone for industrial purposes, but even for jewelry the Diamond has a largely preponderating use. Its points of superiority are its hardness, high refractive powers and hence play of colors, its transparency and its luster. In all these qualities it excels any other known mineral. Hence when in addition to these it exhibits different body colors, as is sometimes the case, no other gem can equal it in value.

Usually the Diamond is colorless or white, although shades of yellow are also common. It is also known in shades of red, green and blue and in brown and black. The two latter are rarely transparent and grade into the varieties known as bort and carbonado, which have no value as gems but are highly important for industrial purposes.

In composition the Diamond is pure carbon, thus not differing chemically from graphite or such forms of carbon as lamp-black, bone-black, etc. It is crystallized, but this can be said of graphite as well. Why carbon should assume the form of Diamond in one case and graphite in another, as well as being amorphous in other occurrences, is not known. Such behavior of a substance is known as dimorphism, and numerous illustrations of it are to be found in Nature.

Being pure carbon, Diamond can be burned in the air. The finely divided dust can be burned in the ordinary blow-pipe flame, and for stones of ordinary size a temperature of about 900 °C is sufficient. The possibility of consuming the Diamond by heat is said first to have been suggested by Sir Isaac Newton, who reasoned from the high refractive index of the stone that it was “an unctuous substance coagulated,” and hence probably combustible. Following this suggestion two Italians, Averani and Targioni, succeeded in 1695 in burning some Diamonds in a furnace, and since then the experiment has been repeated many times. The Diamond does not fuse in burning, but after becoming heated to redness gradually grows smaller, emitting sparks, till it entirely disappears. It leaves no ash except in the case of the impure form known as carbonado. The gas given off has been collected and analyzed and found to be carbon dioxide just as would result from the combustion of other forms of carbon. If protected from the air or free oxygen, the Diamond can be exposed to high heat without change.

Being a crystallized substance and excessively hard the Diamond is usually found in the form of more or less perfect crystals. These have forms such as the cube, octahedron, etc., which belong to the isometric system, and it is in this system that the Diamond crystallizes. The crystals do not possess, however, the highest isometric symmetry, but belong to the class designated by Groth as hexakistetrahedral, being tetrahedral with inclined face hemihedrism. It is very common for the faces to be curved instead of flat and to show etching figures of various kinds. The crystals are often considerably distorted so as to produce pointed and rounded forms, and twin crystals are common. Although so excessively hard the edges of the crystals as found in the beds of streams are often rounded from the wear of the other pebbles, probably chiefly quartz. Only the wear of centuries could produce such a result, however, for, as is well known, it is only with its own dust that the Diamond can be abraded to any appreciable degree by any of the means now used for cutting it.

One important property of crystallized Diamond is that of cleavage parallel to the faces of the octahedron. This cleavage is of much service in preparing the gem for cutting, as by taking advantage of it, broad, flat surfaces can be obtained without grinding. This property also distinguishes Diamond from quartz, for which its crystals as found in sands are sometimes mistaken. Quartz has no cleavage. The fracture of the two minerals is the same, however, being conchoidal.

The massive forms of the Diamond known as bort and carbonado possess little or no cleavage, thus increasing their value as abrasives and for setting in drills, saws, etc. The true bort occurs as rounded forms made up of a confused aggregate of crystals and is harder than ordinary Diamond. Fragments of crystals of no value as gems or any crude Diamond dust are also known as bort in trade. Carbonado is a name given to black Diamond which has more or less crystalline structure. This graduates into the crystallized mineral. Either of these is more valuable than the crystallized Diamond for industrial purposes, although of no value as gems.

As already noted, Diamond occurs of various colors, about half the stones found being tinged to some degree. If the color is but slight, the stone is considered less valuable than if perfectly colorless, but a Diamond of pronounced color is the most valuable gem known.

Among colors of Diamonds, blue is the rarest. The largest and most valuable colored Diamond known is the Hope Blue, weighing 44½ carats. This is valued at about one hundred thousand dollars. It has a brilliant deep blue color and is without a flaw. A deep blue Diamond weighing 67-²/¹⁶ carats was long worn in the French crown, but it was stolen in 1792 and has never been recovered. Red Diamonds vary in hue from ruby red to rose, the latter being the most common. No large red Diamonds are known, the largest being one of 32 carats in Vienna. Another famous one is that in the Russian treasury, for which Paul I paid one hundred thousand roubles. It is of a ruby color. The finest green Diamond known is the “Dresden Green” preserved in the Green Vaults of Saxony. It was purchased by August the Strong in 1743 for sixty thousand dollars. It is apple green in color and weighs 40 carats. Diamonds of yellow color are comparatively common, many of the Cape Diamonds being lowered in value by possessing a yellow tinge. It is said that this injurious yellow tinge can be overcome by dipping the stone several times in a solution of potassium permanganate, the violet color of the latter neutralizing the yellow of the Diamond. The yellow tinge usually also disappears in artificial light. Of large Diamonds possessing a yellow color the Florentine and the Tiffany are the best known. The color of colored Diamonds is generally permanent, but that of some is said to fade on exposure to light. It can also be destroyed or changed by heat.

The luster of the Diamond is a peculiar one, and such as is possessed by few other minerals. In reference to its occurrence in the Diamond it is known as the adamantine luster. It combines the peculiarity of an oily luster with that of glass and that of a metal. It is doubtless due to the high refractive power of the mineral, which causes more than the ordinary number of rays of light to come to the eye. In the impure forms of Diamond the greasy or oily luster becomes more pronounced. Once the eye becomes accustomed to the peculiar luster of Diamond the stone may easily be distinguished by it from glass or minerals with a vitreous luster, such as quartz. Certain other minerals, however, such as cerussite, zircon, and to some extent sphene, exhibit the adamantine luster. In the glass known as strass, used to make imitation Diamonds, the adamantine luster is well imitated.

Diamond is usually transparent, but it may be translucent and even opaque, especially the black varieties. Even otherwise transparent Diamond often contains inclusions which cloud and interrupt its clearness. These constitute the “flaws” which so often injure the value of a Diamond and prevent it from being of the “first water.” These inclusions may be simply small cavities, sometimes so numerous as to make the stone nearly black, or they may be particles of other minerals, such as chlorite, hematite or carbonaceous matter. If the latter, the flaws can sometimes be burned out by careful heating.

As already remarked, the refractive power of the Diamond is very high. The rays of light entering it are bent at a high angle, causing a large degree of what is called total reflection within the stone. The effect of this is to light the stone’s interior. Moreover, the rays of light are concentrated on a smaller part of the surface than is the case with less highly refracting minerals and thus also internal illumination is produced. The most important result of the high refractive power of the Diamond is the wide dispersion of the spectrum, causing the red rays to be widely separated from the blue rays and strong lights of one color to be transmitted to the eye as could not be the case were the different rays less widely separated. It is this power of flashing different colored lights which gives the Diamond one of its chief charms. The index of refraction ranges from 2.40 for the red rays to 2.46 for the violet rays. Ordinary glass has an index of refraction for the red rays of only 1.52 and for the violet 1.54, making the spectrum only about half as long as that produced by the Diamond.

Another pleasing property of the Diamond is the fact that it is usually more brilliant by artificial light than by natural, although some individual stones have a reverse behavior.

Diamond is much the hardest substance known in Nature, and as the proverb says only the Diamond is able to “cut Diamond.” It is ranked 10 in the scale of hardness on which minerals are classified, corundum being the next below it. It is really separated by a wide gap from the latter mineral, however, and its hardness is as much greater than that of corundum as that of corundum is greater than that of the first mineral in the scale. This hardness of Diamond affords a ready means of identifying it, as it will scratch all other substances. It is popularly supposed that Diamond is the only mineral which will scratch glass to any extent, and a stone found is often reported to be Diamond because it will do this. As a matter of fact, however, all quartz will scratch glass and the harder minerals, garnet, topaz, beryl and others will do so easily. Minerals which will scratch glass are therefore common. The Diamond cuts glass instead of scratching it, and is the only mineral that will do this. Although the Diamond is so hard, it is not tough, and can be easily broken with the blow of a hammer. It was a tradition of the ancients that if a Diamond were put upon an anvil and struck with a hammer, both hammer and anvil would be shattered without injuring the Diamond in the least. One occasionally hears this statement made even at the present day. It is entirely untrue, however, the Diamond being as brittle as at least the average of crystallized minerals. The specific gravity of the Diamond is about three and one-half times that of water, determinations showing variations between 3.49 and 3.53. Carbonado is lower, ranging between 3.14 and 3.41. Diamond is thus a comparatively heavy mineral, the only ones among the gems which much exceed it in specific weight being hyacinth, garnet, ruby, sapphire and chrysoberyl.

Diamond becomes strongly electric on friction so that it will pick up pieces of paper and other light substances. It does not retain its electricity long, however, usually not over half an hour. It is not a conductor of electricity, differing in this respect from graphite, which is a good conductor. Diamond becomes phosphorescent on rubbing with a cloth, giving out a light which is visible in the dark. Some stones emit such a light after being exposed to the sun’s rays for a time, as if they took it up from the sun and gave it out again. This has often been stated to be a property of all Diamond, but this is not true, only certain stones exhibiting it. As first suggested by Mr. Geo. F. Kunz, it is probable that this phosphorescence is due to minute quantities of hydrocarbons which emit light on being heated by the friction given the stone. It is curious to note that the light is in some cases given out only from certain crystal faces of the stone. Thus Diamonds are known which give out light from the cubic faces but not from the octahedral, while others are reported as giving out light of different colors from different faces.

The name Diamond comes from the Greek adamas, which means unconquerable. This term was doubtless applied because of the great resistant power assigned to it by the ancients. Besides the well known tradition that it could not be broken by hammer and anvil, they believed that it could be subdued or broken down only when dipped in warm goat’s blood. Our words adamant and adamantine are also derived from adamas, the latter term still being used to describe the luster of the Diamond. The change of adamas into the word Diamond is thought by some to have come from prefixing to it the Italian diafano, transparent, in allusion to its possessing this property.

According to classical mythology the Diamond was first formed by Jupiter, who turned into stone a man known as Diamond of Crete, for refusing to forget him after he had ordered all men to do so. Many medicinal virtues were ascribed to the Diamond, it being regarded as an antidote for poisons and a preventive of mania.

The world’s supply of Diamonds has come almost wholly from three countries – India, Brazil and South Africa. Up to the beginning of the eighteenth century India was the only source of Diamonds known. The Diamond fields of India occur chiefly in the eastern and southern portions of the peninsula. The famed region of Golconda is in the southern part. This is the territory whence have come the most celebrated Indian stones, such as the Kohinoor and the Hope Blue. The French traveler Tavernier reported when he was there in 1665, that sixty thousand men were then employed in these mines. Now the mines have all been given up and the region is abandoned.

The present yield of Indian Diamonds comes almost wholly from mines in a district south of Allahabad and Benares. The Diamonds occur here, as universally in India, in a conglomerate or sandstone made up of the remains of older rocks.

The mines are worked almost wholly by natives of the lower caste, attempts of Europeans to conduct the mining not having met with success. The natives separate the Diamonds by washing, or where the rock is too hard for such methods, break it up by heating and throwing cold water upon it. The production of Diamonds from all of India is at the present time very small, not reaching a million dollars a year in value. It is likely in time to disappear altogether since most of the old mines have been abandoned and even their location forgotten and the returns from the present mines are not very profitable.

The Brazilian Diamond fields were the first important ones to become known after those of India. Diamonds were first found here in 1729 in river sands which were being worked for gold by adventurers who penetrated into the region from the coast. The gold miners paid no attention to the bright crystals sometimes seen in the bottoms of their pans, but a monk who had seen Diamonds mined in India recognized them as gems indeed. While for many years the Diamonds obtained came wholly from the river sands, later, upland deposits were discovered which now afford a part of the supply. Diamonds have been found in the following provinces of Brazil: Bahia, Goyaz, Matto Grosso, Minas Geraes and Parana. In all except Bahia and Minas Geraes the mining is desultory and consists simply in washing river sands by means of wooden bowls. Enough Diamonds are thus obtained to afford a precarious living to the garimperos, as they are called, who follow this occupation. The chief Diamond bearing region of Brazil at the present time is in the province of Minas Geraes, centered about the city of Diamantina. The black variety of Diamond known as carbonado comes chiefly from the province of Bahia and is in large demand for industrial purposes. The Brazilian Diamonds are as a rule small, but exceed all others in luster. The largest Brazilian Diamond known is that named Star of the South, which weighed in the rough 254.5 carats and was valued at one hundred and seventy-five thousand dollars.

As is generally known the chief source of Diamonds at the present time is South Africa. As in Brazil, Diamonds were first discovered here in the river sands and these still afford a small supply. These were first known in 1867, but in 1871 the deposits in place near Kimberley were found and these constitute today the world’s great Diamond mines. The mines now being worked are four in number, and all occur within an area hardly three miles square. Geologically the formation seems to be that of a filling of old volcanic necks by an influx of mud from below. It is this mud which now considerably hardened contains the Diamonds. The largest Diamonds of the world have been obtained from these mines, some exceeding the Kohinoor in size. Their quality is also generally good, although sometimes injured by a yellow tinge.

Besides the above countries, Diamonds have been found in Australia, the Ural Mountains, British Guiana and the United States. The finds have usually been in the beds of streams and are not of sufficient abundance to make systematic mining profitable. The localities where Diamonds have been found in the State of Wisconsin, in this country, are on the terminus of a moraine which came from the North, somewhere in the region of Hudson’s Bay. It is hence not improbable that the “mother lode” will some day be found there.

Finally it is interesting to know that Diamonds occur in meteorites, and hence doubtless exist in other worlds than ours.

Oliver Cummings Farrington.