Diamond Buying Guide

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.

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.

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.

DIAMOND 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.

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.

JUDGING A DIAMOND’S QUALITY

Only high-quality diamonds are suitable for use as gems. In judging the quality (and therefore the value) of a cut diamond, a buyer must take into account four criteria, known as the “four C’s”: color, clarity, carat weight, and cut. Colorless stones are extremely valuable, while yellow or brown-tinged stones are regarded as imperfect. Fancy, colored diamonds, or fancies, exhibit clear, strong colors such as blue, green, red, and orange. Fancies are quite rare and highly prized. The presence or absence of internal blemishes and flaws determines clarity. Weight reflects a diamond’s size. The unit of weight usually employed for diamonds and other gems is the metric carat, which is equal to 0.2 g (about 0.007 oz). Another unit used to express the weight of diamonds is the point, equal to 0.01 carat. A stone of 82 points would therefore weigh 0.82 carat. A 5-carat stone is worth more than five 1-carat stones that are otherwise of the same quality. The final criteria buyers use in determining the quality of a diamond is its cut. The cut is the shape and proportion of the stone, as determined during the diamond-cutting procedure.

SYNTHETIC AND IMITATION DIAMOND

High demand for diamonds has led to the development of methods for producing artificial diamonds. Artificial diamonds used in industry are generally known as synthetic diamonds; artificial diamonds used for ornamentation are called imitation diamonds. Even though the majority of natural diamonds are industrial grade, only about 10 percent of the diamonds used for industrial purposes are natural diamonds. The other 90 percent are synthetic. The two most common processes of synthesizing diamond are the high-temperature high-pressure (HTHP) and chemical vapor deposition (CVD) methods. The HTHP method converts carbon to diamond at high temperature and pressure using a molten metal catalyst. The HTHP method is sometimes also used to change or enhance the colors of some rare natural diamonds, thus making them more valuable on the market. The CVD method produces diamond coatings by heating a hydrocarbon gas over a metal surface. These diamond coatings greatly extend the lifetimes of precision dies, drills, and saw blades.

The high price of gem diamonds has created a market for imitation diamonds made from less expensive materials. Minerals that strongly disperse light, including cubic zirconia, transparent quartz (rock crystal), synthetic rutile, corundum, spinel, and moissanite (silicon carbide), are all used as imitation diamonds. Other imitations are made from a lead glass known as paste or strass. Devices called thermal conductivity probes can detect imitation diamonds because imitation diamonds generally do not conduct heat as well as real diamonds. Imitation diamonds can also be identified optically and are easily scratched by real diamond.

FAMOUS DIAMONDS

A number of individual diamonds have become famous, primarily for their great size but also for their exceptional color, cut, uniqueness, or history. The Great Mogul diamond, reputed to have weighed 240 carats when cut, has disappeared since it was described by the French traveler Jean Baptiste Tavernier in India in 1665. Some authorities believe that the Koh-i-noor diamond, which now weighs 106.1 carats and is one of the British crown jewels, was part of the Great Mogul. Jean Baptiste Tavernier is also associated with the Hope diamond, a 45.52-carat blue diamond that originally weighed 110.5 carats. The Hope diamond is a recut version of the Great Blue diamond. The Great Blue diamond was once part of the French crown jewels.

The Cullinan, the largest rough diamond ever found, was discovered in the Premier Mine in South Africa in 1905. The government of the Transvaal, a British crown colony that later became part of the Union of South Africa, presented the Cullinan to King Edward VII. The Cullinan weighed 3,106 carats (1.37 lb) before cutting and was pronounced by crystallographers to be a cleavage fragment of a considerably larger stone. When the stone was cut a total of 105 gems were produced weighing 1,063 carats in all. The largest of these was a 530.2-carat drop-shaped stone called the Star of Africa or Cullinan I. The Star of Africa is the second largest cut diamond in existence and is set in the British royal scepter. The largest known cut diamond is a yellowish-brown stone called the Golden Jubilee. It was given to the King of Thailand in 1997 to honor the 50th anniversary of his coronation.

The Vargas diamond, found in Brazil in 1938, weighed 726.6 carats in its uncut state. When cut in 1945, it yielded 29 stones with a total weight of 411 carats. In 1934 a diamond of almost precisely the same weight, the Jonker diamond, was discovered in an alluvial deposit near the Premier Mine. The Jonker is the finest-quality large diamond ever found. It was cut into 12 gems ranging from 125.4 to 5.3 carats in weight. In 1967 the Lesotho diamond was discovered, also in South Africa. It weighed 601.3 carats uncut. Other famous diamonds include the Regent, the Sancy, the Tiffany, the Orlov, and the Dresden Green.