Diamond

I

Introduction

Diamond, mineral form of the element carbon, valued as a precious stone. Diamond is the hardest natural mineral and has many other exceptional properties that collectively make it an important industrial and scientific material. Unique geologically, diamonds form at great depths within Earth and are typically billions of years old.

II

How Diamonds Form

Diamonds are crystals composed of pure carbon. In nature, diamond crystallizes from hot carbon-rich fluids. This crystallization requires tremendous heat and pressure—1000 to 1200°C (1800 to 2200°F) of heat and 50 kilobars of pressure. (One bar is based on the pressure the atmosphere exerts at sea level, about 1.02 kg per sq cm, or 14.7 lb per sq in; 50 kilobars is 50,000 bars.) The pressures and temperatures at which natural diamond forms only occur deep underground. Scientists believe that diamonds form at depths greater than 150 km (93 mi), and there is evidence that some diamonds formed as deep as 670 km (420 mi) beneath Earth’s surface.

Concentrations of diamonds great enough to be economically feasible for mining are usually found in Earth’s oldest continental regions, called cratons. Cratons form the cores of most continents and consist of inactive geological areas more than 2 billion years old with thick crust and deep roots extending into the mantle beneath. Craton conditions are ideal for diamond formation and preservation. Scientists have determined the ages of some diamonds by dating mineral impurities trapped within the diamonds. These data reveal that most cratonic diamonds are ancient, some older than 3 billion years.

Much younger volcanic rocks—kimberlites and lamproites—pass through the cratonic rocks in a liquid form called magma during their rapid ascent to Earth’s surface. These flowing veins of rock act as carriers of diamonds and other rock fragments. After eruption they solidify, forming funnel-shaped kimberlite “pipes.” These pipes are primary diamond deposits. Many diamonds are recovered at a distance from their primary deposits in secondary alluvial deposits, which are loose eroded materials left behind by flowing water. In some instances diamonds are also found in sandstones, conglomerates, and other sedimentary rocks that presumably solidified from former alluvial deposits. Wind and glaciers can also transport diamonds from their point of origin at Earth’s surface.

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Small, generally low quality diamonds form in rocks at shallower depths under pressure conditions that are higher than usual for those depths. Tectonic movement, rather than magma, transports these diamonds to Earth’s surface. Deposits of this type occur in areas such as Kazakhstan and typically involve the collision of a continental and an oceanic plate followed by rapid uplift of deeply buried rocks. Diamond deposits brought to the surface by tectonic movement are generally younger than kimberlitic diamonds, and typically consist of microdiamonds (less than 1 mm across) or graphite relics of larger diamonds.

Diamonds are also found in meteorites and near meteorite craters on Earth’s surface. Extremely small diamonds (nanodiamonds) occur in many types of meteorites and have a lower density than other diamonds. Meteorites can also produce pressure and heat at the moment of impact sufficient to transform carbon into diamond. Diamond found in a type of meteorite called ureilite is thought to form directly from graphite contained in the meteorites upon impact. Impact-crater diamonds are opaque and range from very small to around a centimeter in diameter.

Black diamonds called carbonados are thought to have an extraterrestrial origin, as well. These diamonds are only found in 1.5-billion-year-old geologic formations in Brazil and in the Central African Republic, regions that were connected as part of one land mass at the time. Carbonados are not associated with volcanic rock or other types of diamonds. A study published in 2006 established that their chemistry is unlike that of diamonds formed on Earth. Infrared analysis of the carbonado samples indicated that the minerals may have formed in a supernova explosion between 2.6 billion to 3.8 billion years ago. The black diamonds might have come to Earth inside meteorites.

III

Properties

Diamond is the hardest natural substance known. This hardness is exhibited in diamond’s resistance to scratching and its ability to scratch other materials. Steel and glass, for instance, can be scratched by diamond. The Mohs hardness scale, devised by the German mineralogist Friedrich Mohs to indicate relative hardness of substances on a rating scale from 1 to 10, assigns diamond a value of 10. Diamond’s hardness is not a constant quantity but varies even within a single diamond.

Diamonds are crystals composed of carbon atoms. Atoms in a crystal are arrayed in a regular repeating pattern. A crystal’s outward form, bounded by smooth plane surfaces that meet at predictable angles, reflects this internal order. Crystals tend to cleave, or split, along lines called cleavage planes between layers of atoms. In the case of diamond crystals, each carbon atom is bonded to four surrounding carbon atoms. This microscopic arrangement determines the visible shape of diamond crystals, which are generally octahedrons (solid shapes with eight faces). Individual diamond crystals therefore cleave cleanly along planes parallel to the faces of an octahedron.

Two important properties, brilliance and fire, contribute to diamond’s beauty. Brilliance is the fraction of the light that falls on a diamond that the diamond returns to the eyes of an observer—the more light returned, the higher the brilliance. Diamond’s brilliance arises from its index of refraction, which determines the angle at which light is bent as it crosses the boundary between the air and the stone. Fire is the ability of a substance to split white light into rainbow colors—the greater the separation between colors, the greater the fire. Diamond’s fire originates with its dispersion, which is the difference in diamond’s index of refraction for light of different colors. Diamond has both a higher index of refraction and a higher dispersion value than any other natural, transparent, colorless material.

Diamonds exhibit a wide range of transparency and color. Transparency is a measure of the amount of light that passes through a diamond rather than being absorbed. Colorless diamonds, known as white diamonds, are most familiar, but green, blue, red, orange, yellow, and brown diamonds also are known. Structural imperfections or dislocations and the presence of trace elements, mainly nitrogen, cause color in diamonds. Some diamonds luminesce (emit light) when exposed to sunlight or other ultraviolet-light sources. The light the diamonds emit is usually light blue, but yellow, orange, and red luminescence occurs in some stones.

Most diamonds used as gems are single crystals large enough to be easily visible to the eye. Diamond also occurs, however, in polycrystalline forms commonly known as ballas, bort, and carbonado. Ballas is a compact, spherical mass of tiny diamond crystals of great hardness and toughness. Bort is an extremely hard, dark, imperfectly crystallized diamond. The term bort sometimes is also applied to minute fragments of gem diamonds. Carbonado is an opaque grayish or black form of diamond that consists of microscopic crystals and has no cleavage. Ballas, bort, and carbonado are all used industrially, in lapidary (gem-cutting) work, and as a tough coating for the tips of drills and the edges of cutting tools.

Other characteristics of diamonds are frequently useful in identifying the stones and in differentiating between true diamonds and imitations. Because diamonds are excellent conductors of heat, they are cold to the touch and are sometimes called “ice.” Most diamonds do not conduct electricity well, but diamonds do become charged with positive static electricity when rubbed. Diamond resists attack by acids or bases. Since diamonds are a form of carbon, like coal, they will burn, but only when heated to extremely high temperatures.

The density of diamond ranges between 3.15 and 3.53 g/cm³, but the density of pure diamond is always very close to 3.52 g/cm³. Diamond is much denser than crystals composed of elements of similar weight to carbon atoms because the carbon atoms in diamond are packed tightly together. Quartz, for example, is composed of atoms of silicon and oxygen, both of which are heavier than carbon atoms. The density of quartz, however, is only 2.65 g/cm³.

IV

Diamond Cutting

Rough diamonds are not brilliant and can appear greasy. Diamond cutting encompasses a number of processes that bring out the beauty of gem diamonds. These processes include cleaving, sawing or laser cutting, and polishing. A diamond cutter seeks to enhance the brilliance and fire of each stone and to eliminate imperfections, such as cracks and cloudiness. The cutter develops a plan that will accomplish these goals while still producing a gem of the greatest size and hence maximum value. About half of a natural diamond’s size is lost in diamond cutting.

Examining the stone is the first step in diamond cutting. The cutter determines where cleavage planes lie and decides how the stone can best be divided by cleaving and sawing. Ink marks on the rough diamond serve as a guide for the shaping to follow.

The cutter next places the diamond firmly in a holder for cleaving. A light blow of a hammer on the cleaving iron, which is held against the diamond parallel to the cleavage plane, cleaves the stone. In present-day practice cutters more often saw diamonds or cut them with a laser rather than cleave them. The saw is a thin metal disk, the edge of which is impregnated with a mixture of diamond dust and oil.

Polishing, the final step in the cutting of a diamond, consists of forming the facets of the finished stone. Cutters most often choose the “brilliant” form, which has 58 facets. During the polishing process a mount called a dop firmly holds the gem. A flat, horizontally revolving cast-iron wheel coated with a mixture of diamond dust and oil forms the facets. The cutter holds the stone in its dop against the surface of the wheel until the facet forms. In the course of polishing, the cutter moves the stone many times in its dop to present new surfaces for polishing. See also Gemstones.

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