Diamond

I

Introduction

Diamond, crystal form of pure carbon, hardest of all substances, prized as a precious gemstone. The English name is derived from the Latin word adamas meaning “invincible”. Totally colourless diamonds are very scarce; most contain varying traces of yellow or brown. Yellow is caused by the presence of nitrogen within the crystal structure and brown is associated with plastic deformation. Grading diamonds for colour is done by specialists using comparison sets of defined “master stones”. The record auction price bid for a colourless diamond, graded as the top, very rare white (“D” colour) and internally flawless gem was made in 1995. The Star of the Season, a pear-shaped diamond of 100.10 carats, was sold for US$16,548,750. “Fancy” colours such as fine reds, blues, greens, and pinks command the highest prices per carat. The diamond symbolizes love, endurance, and lasting value.

Natural diamonds too inferior to cut are known as “boart” (in the United States, “bort”) and are used in industrial tools, or as “carbonado” if composed of a mass of minute crystals, which is crushed into abrasive grit. Vast quantities of man-made diamonds are also manufactured synthetically for use in modern technology and in numerous industrial applications.

II

Properties

Like graphite and charcoal, which is non-crystalline, diamond is an allotrope of carbon. It is the structure of its crystal lattice and the uniform bonding of the atoms within that together produce its exceptional optical and physical properties. Its scratch hardness is beyond that of all other materials. On the Mohs scale of hardness, diamond measures 10. Measured on a sclerometer, which moves a diamond across a surface under pressure until a scratch is produced, diamond is shown to be 140 times more scratch resistant than corundum (ruby and sapphire), regarded as the next hardest gemstone. In fact, only diamond will scratch diamond.

Another important physical property different from hardness, although often confused with it, is toughness. This is the ability to resist disruption under pressure. A diamond in a vice will withstand extreme pressure and puncture the steel jaws. Indentation tests indicate that to dent a diamond requires a pressure of 8,000 kg/mm², whereas corundum will be dented under a pressure of only 2,200 kg/mm². However, diamond is not so resistant to shock. It can shatter under a heavy blow from hard materials.

Diamond crystallizes in the isometric system with octahedra (eight faces) and rhombic dodecahedra (12 faces) shapes commonly occurring, but several other crystal types are known. Among these are cubes, variations of octahedra—including the hexoctahedron (48 faces)—twin crystals, and maccles. The latter are octahedrons where one half of the crystal lattice is rotated through 180 degrees relative to the other. In processing “cuttable” diamonds, such knowledge has practical implications; cleaving can only be done cleanly along planes parallel to octahedral faces.

Important optical properties of a polished diamond—other than its colour—are seen in its high refractive index (2.4175), high degree of clarity, colour dispersion, reflectivity, adamantine lustre, and scintillation. The total effect, known as brilliance, depends entirely on the diamond’s “life” and “fire”. The purpose of “life” is to return a maximum 83 per cent of light back to the eye, without allowing it to escape through the diamond’s back or sides. “Fire” is the display of spectrum colours caused by white light reflecting internally, before it is refracted back to the eye. When either the diamond is moved or its observer moves, “fire” describes the darting flashes of spectral colours that are visible.

Many diamonds exhibit fluorescence when exposed to sunlight or ultraviolet light. The colour is usually light blue, but yellow, orange, green, milky-white, and red fluorescence may occur in some gems. Under X-rays, a bluish fluorescence becomes evident and this feature is often now used in their recovery from ore at the mines.

When photographed under X-ray, diamond is transparent and this fact is sometimes used to distinguish it from imitations. While being similar in appearance to diamond in varying degrees, diamond simulants, or substitutes, are made of different natural or synthetic compositions with inferior properties. Notable examples include lead glass or paste, colourless sapphire, and synthetic cubic zirconia.

A simple detection method utilizes diamond’s excellent ability to conduct heat. Diamond feels cold to the touch and dissipates heat very quickly, and so a thermal probe has become an essential part of a jeweller’s equipment. Synthetic moissanite, a new simulant introduced in 1998, possesses similar thermal properties to diamond. However, moissanite is readily detectable as a simulant using standard testing techniques.

Diamonds range between 3.15 and 3.53 in their specific gravity (SG), but the value for cuttable diamonds is usually given as 3.52 SG. This measurement is the ratio of the weight of the diamond to the weight of an equal volume of water. Other physical properties of diamond include its resistance to attack both by acids or alkalis and to heat. (See Industrial Diamond below for the use of these properties.)

III

Formation

Diamonds are often much older than the two host rocks—kimberlite or lamproite— in which they were transported to the surface. Crystallized from pure carbon under great heat and pressure in the Earth’s upper mantle at depths of at least 180 km (112 mi), diamonds may vary in age from between 660 and 3,300 million years (within the Precambrian period). During the past 1,200 million years, when different episodes of volcanic activity occurred, primary diamond deposits—”pipes”, dykes, or flat-lying sills—were emplaced. The Premier Mine in South Africa dates back to the earliest episode while others, such as the Orapa Mine in Botswana, are of a much later period, perhaps 90 million years ago.

Later erosion of the Earth’s surface by rain, sun, and wind released many diamonds from their primary location. Some, remaining relatively close to their host, formed secondary eluvial deposits in soil or surface rubble. Others travelled far greater distances and were dispersed in alluvial deposits, either in ancient riverbeds or spread along beach terraces. Some were carried into the sea and not returned by the surf, becoming marine deposits, having settled on the ocean floor itself.

IV

Occurrence

A “pebble” picked up by a child on the banks of the Orange River in 1866—later identified as a 21 7/8 (old) carat diamond—is said to have been the first step in the discovery of diamonds in southern Africa. The diamond rush to search for alluvial diamonds in the gravel of the Orange and Vaal rivers was greatly accelerated in 1870 and 1871 following the discovery of “dry diggings” in a district near the present-day city of Kimberley. Initially the diggers were searching through a limestone overburden, but within a year a decomposed, yellowish clay (“yellow ground”) was uncovered, and yet more diamonds were found.

As the diggings went deeper, a hard, bluish rock appeared, first causing great alarm, but later proving equally productive. This blue ground, later named kimberlite, was the parent material from which the weathered yellow ground had formed. The solidified kimberlite occurring in pipes resembles cones or carrots while the lamproites so far discovered are said to be shaped more like a champagne glass. Lamproite, a similar basalt rock to kimberlite, but composed of different constituents and indicator minerals, was first identified as another diamond-transporting medium in Australia in the late 1970s.

Of almost 6,000 known kimberlite intrusives in the world, less than 1 per cent are currently being mined, with more than half being classed as small mines. World natural diamond production in 1996 was estimated at 114.5 million carats, with Australia, the Democratic Republic of Congo (formerly Zaïre), Botswana, Russia (Sakha), and South Africa being the volume leaders. In terms of value (determined by the quality of the diamonds that are mined) Botswana is the world’s leading producer today.

Even so, less than 1 per cent of all diamonds produced result in polished diamonds of over 1 carat in size. Other countries where diamonds have been found include India and Borneo (somewhat historically); Bolivia, Brazil, Venezuela, and Guyana in South America; the United States and Canada in North America; Angola, Central African Republic, Guinea, Côte d’Ivoire, Ghana, Lesotho, Liberia, Namibia, Sierra Leone, Swaziland, Tanzania, and Zimbabwe in Africa; China in Asia; and Finland within Europe.

In 2000, at the World Diamond Congress in Antwerp, two of the world’s largest diamond trade groups—the International Diamond Manufacturers’ Association and the World Federation of Diamond Bourses—agreed to a plan to curb the illegal trade in so-called “conflict diamonds” that provides income for rebel groups, particularly in Africa. Under the new proposals, to be implemented by the end of 2001, an international certification system is expected to be set up to confirm the provenance of diamonds, and anyone caught trading in diamonds from non-legitimate sources will face expulsion from the trade.