Introduction to Diamonds

The mined stones are classified into different varieties

1. Diamonds, well crystallized, transparent and gem quality

2. Bort, boart or bortz, poorly crystallized, grey to brown in color, translucent to opaque and used mostly in industry as an abrasive. The large and black stones of granular crystalline structure found almost exclusively in the state of Bahia in Brazil are also known as carbonado or carbonate.

3. Ballas – spherical masses of minute diamond crystals more or less concentrically arranged. The mass in very hard and rough and has important industrial applications.

Rough diamond crystals are mostly of diameter lass than a millimeter in diameter. Bog crystals are extremely rare and so valuable that they acquire the status of an antique and are known by special and distinctive names. History and legend play a bigger part than the actual inherent value of these stones. They are mostly found as part of the crown jewels of various nations.

A natural diamond mines from the earth always has inclusions of graphatite or other minerals that are visible as dark sports in the rough stone. Voids often filled with as are visible as white or shiny spots within the stone. Stones that do not have any inclusions even under 50x magnification are extremely rare and are collectors items.

Carbons atoms arranged in inter-penetrating face-centered cubic lattices form the crystal structure of diamonds. An infinitely large crystal would be totally colorless and transparent except to ultraviolet light of wavelengths shorter than 230nm. However, all natural diamonds contain defects in the crystal lattice as well as impurity elements and these defects cause color in the stone. Again, the growth patterns may abruptly change, with further layers becoming mirror images of the earlier growth. The twining that often occurs within the crystal makes the crystals very hard to cut and they are called macles.

Diamonds have been found all over the world, the most ancient and famous being old Indian deposits that were worked from earliest times. Till about the late 1700s, all the diamonds in the world were found in fields and on river beds, mostly near Hyrdabad in the Deccan plateau. Large stones like the Kohinoor were found there around 1300AD. The town of Golconda so became the center of the diamond trade of the world. In spite of the output from India, Borneo, and to some extent Brazil, diamonds were very rare. Valuable diamond fields in the gold mining area of Minas Geraiswere discovered in Brazil about 1725. Portuguese merchants took Brazillian diamonds to Goa, India, to pass them off as Indian stones. Mining was carried out so intensively in Brazil that the main areas were almost exhausted within twenty years.

At about the time that the Brazilian fields were being depleted, the first diamond field of South Africa was discovered in 1886Ad, when children of a Boer farmer found “a pretty pebble” in the dandy bed of the Vaal River. Four years later, diamonds were found in the earth far from a river source, and the practice of dry-digging for diamonds was born. More sophisticated mining techniques allowed deeper subterranean digging, as well as more efficient river marine mining than ever before. By 1871 world annual production, mostly from South Africa, exceeded a million carats. Soon South Africa had the monopoly for diamond production, until major deposits ere found in Siberian permafrost in 1954. Currently Western Canada is the site of the world’s newest diamond rush. Diamonds have also been found in Urals and in Australia and these sources compete with South Africa.

Diamonds are extensively used in industry, but are better known for their use as gemstones. The value of a diamond depends in size, quality and shape. In the seventeenth century, Tavernier recorded that the value of a diamond was proportional to the squire of the weight, and this is still true. The most popular cut as a gemstone is the brilliant, a round cut with 58 facets that was developed to obtain the maximum brilliance and fire from the stone. There are also other fancy cuts like the kite, triangle, and baguette. The term melee refers to stones cut from small fragments of larger rough stones, obtained after cutting. Approximately 8-16 such stones together weight one carat, and many of these small stones are cut with 58 facets. Similar melee stones are cut with only 18 facets and maybe as small as 0.01 carat each.

Merchants found that the dried brown fruit of the locust tree, which looked like a horn, had seeds of an extraordinary uniform weight. In course of time, the seed, the keration or carat became the standard for weighing diamonds and gold. Now the standard weight for diamonds is the carat (200mg), rather than the more picturesque seed.

When heated in oxygen above 650C, a coating of graphite forms on the diamond. In an inert atmosphere, the transition to graphite occurs above 1570C. Diamond is extremely inert to acids and chemicals until heated to 1020K, but sodium nitrate attacks it at 430C and metals react to form carbides. At pressures of 70,000 atmospheres and 2500C, graphite may crystallize into diamond. The specific gravity of diamond is 3.5 and its refractive index is 2.4 as against 1.5 for glass and 1.33 for water. Diamond also has high dispersion and this gives fire to the crystal. Diamond conducts heat but is a good electrical insulator; this is a sure way to detect a real stone from fakes.

Almost all diamonds contain impurity atoms, mainly nitrogen and boron, but these cannot be seen even at the highest magnification. The effect of these impurities is to change the properties of the stone, mostly by giving it a light color. For instance, the nitrogen tends to collect into small clusters of two to three atoms and the interactions between the carbon and nitrogen atoms create new energy levels in the crystal. An optical center is formed that absorbs light of wavelengths longer than 230nm. Natural diamonds may have several such optical centers of different configurations. When a single nitrogen atom substitutes for a carbon atom, an N3 optical center is formed and the ultraviolet absorption edge comes close to the visible region. The “A” optical center produced when nitrogen atoms replace 2 adjacent carbon atoms gives substantial infrared absorption in the region 10,000 to 7000nm. An arrangement of 4 nitrogen atoms around a missing carbon atom gives a B center with the result that the ultraviolet absorption is lowered through the infrared spectra of A and B centers as similar. The addiction of a nearby vacancy in the carbon lattice results in the so-called H3 and H4 centers.

Natural diamonds contain these optical centers in randomly variable concentration and gemologists label the type of diamond according to the absorption spectrum. Most diamonds have N3, A and B centers and these are the Type I sort. Type Ia diamonds contain primarily A and B centers, but if N3 centers are predominant, it becomes Type Ib. the Type Ia is further divided into Type IaA and Type IaB according to whether there are greater A or B centers. Type II diamonds have little or no nitrogen in them but may contain boron that makes them electrically conducting; these are the somewhat rare type IIb semi-conducting stones. In nature, it is possible to find stones which are of more than one type. Synthetic diamonds are usually Type Ib and may have up to ten percent of iron and nickel as contamination from the process. Ultraviolet, X-rays and electrons excite luminescence in diamond that is usually blue though often yellow and green colors are seen.

In general though, the color of diamond is not directly related to the impurities and their concentrations but us due to defects produced in the lattice by the impurity atoms. Bombardment of the crystal by atomic particles can knock out carbon atoms from their positions in the crystal lattice, produce defects and so induce color. Gamma rays and X-rays do not have any mass and so cannot displace the carbon atoms and thus do not produce defects.

Exposed to a stream of high energy electrons, the diamonds attain a blue shade, the depth depending on the penetration of the electron into the stone. Electron-hole pairs are trapped at the defects that absorb red light and so color the stone blue, but this color fades with gentle heating. A high energy electron colliding with a carbon atom could give it enough energy for it to knock other carbon atoms out of their places. For example electrons of 13 MeV from a big accelerator can give a carbon atom a maximum energy of 35 KeV; this atom is capable of displacing about a hundred other atoms. This cascade of collisions tends to make the diamond greenish in color. Neutrons and alpha particles are far more effective in transferring energy to the carbon atoms and the cascade of collisions turn the diamonds into a definite green color. Even the diamond only a small distance and the color is a surface effect that may disappear when the stone is polished.