Diamonds are the mineral form of carbon. They are valued as precious stones and are used for various industrial purposes. Diamonds occur in various forms, including the diamond proper, bort, ballas and carbonado. Bort is an imperfectly crystallized type of diamond, very hard and dark in color. Ballas is a compact spherical mass of tiny diamond crystals of great hardness and toughness. Carbonado, sometimes called black diamond or carbon, is an opaque grayish or black form of diamond with no cleavage. Carbonado, ballas and bort are all used industrially in lapidary work and for the cutting edges of drills and other cutting tools.

Diamond is the hardest substance known and given a value of 10 in the Moh's hardness scale, devised by the German mineralogist Friedrich Mohs (1773-1839) to indicate relative hardness of substances on a rating scale from 1 to 10. Its hardness, exhibited in its resistance to scratching, is not a constant quantity, but varies in every diamond with the crystallographic direction, being greater on surfaces parallel to the octahedron than on those parallel to the dodecahedron planes. Hardness on the same face or surface varies with the direction of the cut. Ease of cutting is affected also by the presence or absence of crystalline twinning in a diamond. Diamonds from river beds are said to be harder than those from pipe mines; such diamonds are in fact not harder, but tougher, that is, they show greater resistance to breakage, as in the course of being transported by water, stones with small cleavage cracks or inclusions tend to break along the lines of weakness so that the resultant fragments are virtually free of structural flaws.

The crystallization of the diamond is in the isometric system. Octahedrons and rhombic dodecahedrons are the crystal forms most commonly found, but cubic and other forms also occur. Rounded, distorted, and twinned crystals are also not uncommon. Crystalline diamonds always cleave cleanly along planes parallel to the faces of an octahedron. In specific gravity, diamonds range between 3.15 and 3.53, but the value for pure crystals is almost always 3.52.

Diamonds exhibit a wide range of transparency and color. All good gem diamonds are transparent, and colorless stones known as white diamonds are very valuable. A yellowish or brownish tinge often occurs and is regarded as an imperfection, although the valuable Tiffany diamond is deep yellow. Brown diamonds are not uncommon. Green and blue diamonds are rarities, and red diamonds are rarest of all. Good quality diamonds of clear, strong and unusual color are highly prized. Color in diamonds is caused by the presence of minor elements other than pure carbon.

Two important characteristics of the diamond when used as a gem are its brilliancy and fire. Both the index of refraction and the dispersion (the physical properties that determine brilliancy and fire, respectively) are higher for diamond than for any other natural, transparent, colorless stone. Uncut diamonds have a greasy luster and are not brilliant, but the same stones when cut exhibit a high luster, characterized technically as adamantine. The effect of the high dispersion is to separate the colored components of white light in a marked fashion so that the stone, properly cut, sparkles with spectral colors.

Other characteristics of the diamond add nothing to its appearance but are frequently useful in identifying the stone and differentiating between true diamonds and imitations. Because diamonds are excellent conductors of heat, they are cold to the touch. Most diamonds are not good electrical conductors and become charged with positive electricity when rubbed. Genuine cut diamonds are transparent to x-rays, whereas imitation diamonds, usually made of lead containing glass are not. Some diamonds exhibit fluorescence when exposed to sunlight or other ultra-violet light sources. The color of fluorescence usually is light blue, but yellow, orange, milky white, and red fluorescence may occur in some stones.

Another important physical characteristic of the diamond is its resistance to attack by acids or alkalis. Transparent diamond crystals heated in oxygen burn at about 800 degrees C, forming carbon dioxide. In the atmosphere the temperature of combustion for diamond varies between 690 degrees C and 875 degrees C.

The exact mechanism of the production of diamonds is still a matter of debate among geologists, but it is certain that both tremendous pressure and heat are required for the crystallization of carbon into this form. Hence diamonds were probably produced in molten rock or magma, in which these conditions prevail, far below the surface of the earth, and were carried to the surface by magma, which later hardened. The parent rock is apparently peridotite, but many diamonds are recovered from alluvial deposits at a distance from their point of origin. In some instances the stones are found also in sandstones, conglomerates, or other sedimentary rocks, which presumably represent alluvial deposits of earlier geologic areas.

Very small and opaque diamonds known as hexagonal diamonds have also been found in certain types of meteorites. Their physical properties are identical to those of natural cubic diamonds but they have a different crystal structure, the layers of atoms being turned sixty degrees from the position in which they are found in cubic diamond. Hexagonal diamonds are formed directly from graphite in meteorites at the moment of impact, during which very high temperatures and pressures up to 15 million pounds per square inch occur for a few millionths of a second.

Diamonds are found widely separated localities in Brazil, one near the city of Diamantina in Minas Gerias, another in Baia, and others in Matto Grasso. The Brazilian diamond workings are most valuable in production of ballas and carbonado.

A "pebble" picked up by a child on the banks of the Orange River in South Africa in 1866 and identified as a 21 carat diamond was the first step in opening the diamond fields of that region, which have become the greatest in the world. The diamond rush to search for alluvial diamonds in the gravel of the Orange and the Vaal Rivers was greatly accelerated in 1870 and 1871 following the discovery of "dry diggings" in the district near present day Kimberley. These diggings were roughly circular patches of yellow clay in which diamonds were found. As the miners dug deeper in the clay, often called the "yellow ground", they found below it a hard, bluish rock, which also proved to be productive. This "blue ground", scientifically identified as kimberlite (a form of peridotite), is the parent material from which yellow ground is formed by weathering. Further mining disclosed that the circular areas of yellow and blue ground are the tops of funnel-shaped "pipes" of kimberlite, which continue downward for an undetermined distance. Similar pipes, not all of which contain diamonds, have been found at other various locations in South Africa. Pipes are believed to be of volcanic origin.

Diamond deposits, most of which are alluvial, have been found in other parts of Africa, including Tanzania, the Republic of Zaire, Ghana, and Sierra Leone. There have been discoveries also in Australia, Borneo, the Ural Mountains, Siberia, Venezuela, and Guyana. Isolated stones have been found at various places in the United States and a there is a kimberlite pipe in Arkansas, which yields diamonds, although not in sufficient quantities for profitable mining.

In India, which for centuries was the only known source of diamonds in the world, present-day production is limited to small quantities of diamonds from conglomerate beds and a kimerlite pipe.

The processes used for mining diamonds in India and later in Brazil were extremely primitive, consisting chiefly of manual washing of alluvial clays and gravels, and breaking up of the conglomerate and other diamantiferous rocks. Similar methods were employed at first in the South African diggings, but since 1889, when the mines were consolidated under the control of a single company, modern mining practices have been introduced.

The original mine workings at Kimberley and the other pipes were open pits, first in the yellow ground and later in the deeper blue ground, from which it became more difficult and expensive to remove the mined material. Eventually a system of underground mining was instituted. The Kimberley mine, the deepest of the South African mines, was worked to a depth of more than 3600 ft before it was closed in 1914.

Chambering is the system of mining most widely used underground. Tunnels are driven across the pipe 40 ft below each other and chambers are excavated on each level below. The blue ground, or kimberlite, is hoisted to the surface and delivered to a crushing station adjacent to the headgear.

A new method known as block caving is employed increasingly. Cone shaped excavations are cut beneath a huge volume of the blue ground 400 to 600 ft in height. As this overhanging mass begins to crumble of its own weight and settle on top of the cones, it breaks into pieces small enough to pass down into tunnels below. There, mechanical scrapers pull it to a wide grating, through which blue ground of suitable size falls into cars. The larger lumps are pulled to an underground crusher and reduced to lumps of less than 1.25 inch diameter. A conveyer belt then carries the blue ground to the shaft, through which it is hoisted to the surface.

In the next phase of the operation the blue ground is washed in rotary pans filled with a washing medium called puddle. The heavier minerals, of which the diamond is the heaviest, settle to the bottom of the pans and are drawn off. The finely divided material from the washing pans is treated in jigs or boxes with screen bottoms through which a pulsating stream of water is passed, and the very small diamonds are recovered by a process of electrostatic separation. The larger material is fed into large cones filled with a mixture of water and ferro-silicon. Diamonds and other heavy materials sink to the bottom of the cone and lighter waste floats off. After further screening and washing, the concentrates are poured on vibrating grease tables over which water flows. Only the diamonds, which are water repellent, adhere to the grease, and the wet gravel particles pass over the grease tables with the water.

The recovery of diamonds from the concentrates of alluvial deposits is similar in princlple with one difference. Alluvial deposits from the marine terraces of South West Africa and the beds of South African rivers have on their surfaces a microscopic film of mineral salts. These soluble salts cause the diamond surfaces to become wet when in contact with water, thus preventing them from adhering to the grease on the grease tables. Consequently, until about 1950 alluvial diamonds had to be hand picked from the concentrates, an inefficient and costly method. In present-day practice the stones are given a water repellent surface by treating the alluvial concentrates with an alkaline solution of oleic acid (whale acid oil) before they are fed to the grease tables. Alluvial diamonds from the rivers of other regions, as in western Africa, are usually free from this coating of mineral salts and do not require special previous treatment before reaching the grease tables.

The elaborate system of concentration used in the recovery of diamonds is necessary, because diamonds comprise an average of only one in twenty one million parts of the blue ground. By contrast the upper limit for the economical recovery of gold is one part in three hundred thousand.