T The Diamond today
1 - Gemstones 2 - Diamond 2.1 – The gemstone of gemstones 2.2 – The name 2.3 – What a diamond is 2.4 – How a diamond is formed 3 - Characteristics 3.1 - The diamond is a mineral. 3.2 - Hardness 3.3 - Cleavage 3.4 – Dispersion and fire 3.5 - Other characteristics 3.6 - Inclusions 4 - Diamond ores 4.1 – Primary ores 4.2 – Secondary ores 5 - A very tiny diamond. 6 - Fancy diamonds 7 - Types of diamonds 8 - Diamond grading 8.1 - Carat 8.2 - Colour 8.3 - Clarity 8.4 - Cut 9 - Diamond substitutes 10 - Synthetic diamonds 11 - Enhanced diamond 12 - The Moissanite story 13 - Diamond grading 14 - Bibliography
Gemstones, bountifully blessed with splendour and colour, seem to be in a world of their own, isolated from the common creations of nature. Since the beginning of civilization, the fascination exerted by these wonderful stones has produced numerous theories on their origins, including the extraordinary idea that they were sexually reproduced. The ancients thought that they had a supernatural origin and attributed all kinds of power to them. It was only in the seventeenth century that, thanks to the English physician Robert Boyle, scientists reached the conclusion that gemstones are a rare and noble product of nature. They belong to the mineral world and are spontaneously formed without the intervention of man, even if many of today’s artificial gemstones are of great value and synthetic stones can sometimes barely be distinguished from natural ones. The so-called genetic processes of minerals, or rather the way minerals are formed, can be traced back to the general genesis of rocks, which are in fact aggregations of minerals. Rocks can be defined according to a simple classification: magmatic (or igneous), metamorphic and sedimentary rocks, even if there are several other terms that can describe the transition from one to another. Gemstones are usually formed in particular environmental conditions beneath the Earth’s crust. The genetic conditions necessary for their formation vary dramatically from one gemstone to another, but almost all of them need a combination of certain constitutive elements that occur in a certain proportion and at a very high temperature and pressure. These elements then cool and solidify at particular speeds for a certain period of time. The probability of all these factors occurring simultaneously is very low, and for this reason gemstones are rare. The distinguishing characteristics of gems are beauty, durability and rarity.
2.1 – The gemstone of gemstones The earliest diamonds came from India. They were extracted from three different alluvial deposits, but the most important extraction area was Golconda. It was not a mine, but rather an important trading centre, whose name was given to an area in the south of the country where many famous stones were produced. Some of these gemstones are still famous today, whereas the fate of others is unknown. Every gem has left a trail of betrayals, intrigues, murders and wars in its wake. The most famous of these ancient diamonds is the Koh-i-Noor which, after being newly cut, weighs 108.83 carats and is part of the British Crown Jewels. The Great Mogul (180 carats), described in detail by the French gem merchant, Tavernier, in the middle of the seventeenth century, was lost after the looting of Delhi in 1739. The Blue Hope (45.52 carats), with its long history of tragedies and disasters, holds a privileged position in the exhibition at the Smithsonian Institution in Washington D.C. In 1725, diamonds were discovered in fluviatile gravels in Brazil, and for 140 years this country became the greatest producer of diamonds in the world. In 1866, diamonds were discovered in South Africa, firstly in fluviatile gravels and later in diamond pipes, the rocks in which they formed. Over the following 100 years numerous deposits of diamonds, both alluvial and primary, were found in a dozen African countries. Notwithstanding all these deposits, South Africa continues to be the most important producer of diamonds in Africa. In fact, the largest gem-quality diamond, the Cullinan (3106 carats), was found in the Premier mine in that country in 1905. In more recent years, many diamonds have come from Canada, Australia and Russia. Although the data is unreliable, it is estimated that Russia produces about 13 % of diamonds worldwide and that this figure is continually rising. In 1995, worldwide diamond production amounted to 108 million carats and availability is increasing. Every year new discoveries are reported and the production in old mines increases. For example Botswana produces about 17 % and Australia about 12 % of diamonds in the world, both as gemstones and as industrial diamonds. About 75 % of diamonds produced are industrial and cannot be used as gemstones.
2.2 – The name The term diamond comes from the Greek word adamas = invincible, a word which was probably used by the Greeks to indicate any hard stone, such as corundum. The first sure reference to diamonds occurs in Latin literature of the first century A.D. The diamonds known to Romans undoubtedly came from India, which was the only country known to export these stones until the eighteenth century.
2.3 – What a diamond is A diamond is the crystal form of carbon, used both as a precious stone in jewellery and for various industrial applications. It occurs in various forms: as well as the Diamond proper, there is the Bort, a variety of imperfectly crystallized diamond, extremely hard and with a dark colour; the Ballas, a compact, spherical mass of tiny diamond crystals; and the Carbonado, sometimes called a black diamond, with an opaque greyish or black shape and no cleavage. Carbonados, Ballas and Borts are used to cut and smooth stones, for the cutting edges of oil drills and in the manufacture of other tools.
2.4 – How a diamond is formed The diamond – pure crystallized carbon – is undoubtedly the gemstone which forms at the greatest depth. In all probability this process starts at 145 - 200 Km below the earth’s surface, in the upper part of the mantle, the area almost 3,000 Km thick situated between the internal nucleus - fused – and the solid external crust. It is thought that the upper part of the mantle is composed of rocks at high specific gravity, rich in iron and magnesium, and in particular of a dark coarse-grind peridotite, interspersed in many areas with molten material called magma. From the results of synthesis trials in laboratories, it has been calculated that temperatures of at least of 1,500°C and pressures of 70,000 Kg/cm2 - 65,000 times the normal atmospheric pressure – are needed to crystallize carbon atoms into their most valuable form. Experts, however, agree only in general terms on the process which leads to the formation of diamonds in the depths of the earth and on how diamonds are transported to the surface. The most reliable hypotheses maintain that most diamonds formed in the bowels of the earth more than 990 million years ago from two kinds of rocks: peridotites and eclogites. They piled up at the foot of cratons over various periods of time – some of them for even 3,200 million years – before being transported to the surface. The two rocks that are usually associated with diamonds, kimberlite and lamproite, were only the mechanism that moved diamonds to the surface and are not in any way related to the formation of diamonds. From isotopic studies (Mottana, 1990) which allow rocks to be dated by means of the minerals contained within them, further astonishing new information has emerged: according to the 40Ar/39Ar report, cubic diamonds from Zaire have been accorded an age of 5.8-6.0 Ga, older than the earth itself (4.53 Ga)! A hypothesis has therefore gained ground which is shattering from a cosmological point of view: trapped within diamonds is a portion of interstellar gas inherited from the condensation process in which the nebula, which later generated our galaxy, gradually became transformed into solid matter. This most recent hypothesis, even though it may seem to be science-fiction, fascinates both diamondologists and cosmologists since, precisely in the last three years, the presence of microdiamonds in meteorites has been ascertained, and it has therefore been possible to confirm their presence in cosmic dust.
3.1 – The diamond is a mineral.
Minerals are uniform compounds of chemical elements and, as well as certain fixed relations that are characteristic of them, most minerals have a precise spatial geometric arrangement of their atoms. This invisible and particular atomic geometry originates one of the most significant characteristics of minerals and, therefore, gemstones too: their crystal structure. The various crystal shapes of gemstones are the external manifestation of a precise internal order: a regular and three-dimensional arrangement of the atoms that form the mineral. Since this arrangement of atoms depends on the properties of the elements present in the mineral, the basic scheme of the crystal structure is a distinctive character of every mineral. In order to classify these different arrangements of atoms, crystallographers have identified the elementary cell, i.e. the smallest geometric shape of interlinked atoms that is repeated billions of times throughout the crystal. The elementary cell of diamonds is a face-centred cube. In the crystal lattice structure of diamonds, carbon atoms are arranged in such a way that each of them is set at the centre of a tetrahedron with adjacent atoms located at its peaks. The minimum distance between the nuclei of the two carbon atoms in the lattice structure is only 0.1542 Å (1 Angstrom = one millionth of a mm). The covalent bond, with such a small radius and compact cubic arrangement, makes the diamond the hardest known material. Gravity varies from 3.15 to 3.53 and in purer crystals is almost always 3.52. The cubic cell imposes very precise and limited shapes of crystal growth: the octahedron is the most common, followed by the rhombicdodecahedron and other more complex shapes. In contrast to the ideal geometric shape, natural crystals are almost always distorted, have rounded corners and peaks, and surfaces that are curvy, twisted or twinned; geometric shapes of growth and corrosion called trigon are very common on the faces and peaks. In the picture, the most common shapes of a rough diamond are shown.
3.2 – Hardness The hardness of a mineral is usually measured using the Mohs scale, in which 10 natural compounds are ordered in such a way that each mineral can scratch the one below it, but cannot be scored by it. The diamond is at the top of the Mohs scale and is indicated by the number 10. It cannot therefore be scratched by any other materials, only by itself. The diamond is at the moment the hardest of known natural and synthetic substances, but at the same time it is extremely fragile, a soft blow from a hammer can break it down into fragments.
3.3 – Cleavage As well as its fragility, one must consider the cleavage, another characteristic of diamonds which envisages the possibility of fractures along precise planes. Cleavages are easily recognisable in almost all crystals and occur along the planes in correspondence with the weaker interatomic bonds. In diamonds they occur only along octahedron planes and are exploited in the cutting of diamonds.
3.4 - Dispersion and Fire Dispersion and fire are two important characteristics of brilliants. They are related to the refractive index and dispersion, which are higher for diamonds than for any other natural, transparent and colourless stone. Dispersion is the diverse refraction undergone by various wavelengths of white light travelling from one medium to another: a colourless material decomposes white light into a spectrum of seven colours. Fire is the brightly coloured effect produced by the decomposition of light passing through a diamond.
3.5 - Other characteristics Other characteristics of the diamond, even if they add nothing to its appearance, are frequently useful in identifying the stone, in testing its authenticity and in distinguishing true diamonds from imitations. Since diamonds are excellent conductors of heat, but not electricity, they are cold to the touch and become charged with positive electricity when rubbed. Unlike imitations, genuine cut diamonds are transparent to X rays and are particularly resistant to attack by acids or alkalies. Transparent diamond crystals heated in oxygen burn at about 800°C, liberating carbon dioxide.
3.6 - Inclusions A perfectly transparent diamond, no matter what colour it is, is a rarity. Transparency is frequently reduced by the presence of inclusions of various types: minerals, cleavage, cracks, and other fractures. It is no wonder that only 25 % of diamonds produced worldwide are suitable for jewellery. Whereas on the one hand microscopic inclusions degrade the appearance of the stone, on the other they provide mineralogists with precious information about the materials that crystallize in the deep bowels of the earth. Under a microscope they appear mostly black, and for this reason used to be identified as graphite. Today, they can be clearly identified with a modern instrument, the electronic microdrill, even if they are only 2 or 3 µm larger. Graphite is rare, and most materials are garnet and pyroxene.
Diamond ores can be primary and secondary.
4.1 – Primary ores In primary ores, diamonds are found mainly in pipes, narrow funnel-shaped conduits of volcanic rock caused by ancient volcanic eruptions that are now distributed for kilometres along the earth’s crust, from the upper mantle to the surface. Crystals emerge still embedded in the rock which transported them towards the surface of the earth’s crust: kimberlite. As well as incorporating solidified magma, kimberlite (composed mostly of forsterite olivine, mica phlogopite and pyrope garnet) contains fragments of rock torn from the walls of the conduit through which it emerged in order to reach the surface and dragged along in its violent progress upwards; kimberlite also exhibits the effects of oxidation and alteration caused by surface water. The conclusion is obvious: kimberlite randomly took on materials as it crossed the mantle, from the peridotic and eclogitic layers which both contain diamonds: they are thus a typical mineral, well-distributed in all the deepest parts of our earth. This is the most reliable hypothesis, in contrast to the idea that diamonds formed inside kimberlite itself from materials that were initially fluid. Whatever the process, it appears to be no longer active. For 15 million years, no further pipes have been formed.
4.1 –Secondary ores The formation of secondary ores is instead related to the metamorphosis and disintegration of rocks containing diamonds, caused by atmospheric agents (sun, rain, change in temperature, wind) over millions of years; to the transportation of rocks broken up by rivers and the wind; and to their being deposited in areas sometimes very far from their places of origin. These are thus true alluvial and Aeolian deposits that formed in valleys dug out by water, within river creeks where the current was weaker, on desert plains where winds could form dunes and even in the sea, at the mouths of rivers of streams which flowed and still flow through diamond areas (Gumey et al. 1991). From the distant times of diamond extraction in India until the discovery of vast ores in South Africa, only secondary ores were known and exploited. Elementary tools were used for extraction: picks and shovels were used to remove huge quantities of agglomerates, rubble and sand, which was then washed and sifted and, finally, the diamonds were recovered by hand. In order to have access to diamond creeks, streams and rivers were even diverted and this water was used in the washing process. The same procedure was used in South Africa until 1890, when it was realised that certain ores extended in depth: this was the first time in the long history of the diamond rush that a primary ore was discovered. The discovery was made in a town that would then become the city known today as Kimberley. Diamonds in that area lay in yellow, eroded and muddy ground that, at 20-40 metres in depth, changed into a compact, bluish-black rock; at an even deeper level, around 500 metres, the original diamond rock was later identified, then given the name of kimberlite. Discovered in Brazil, Canada, Africa, India, Siberia, Australia and China, kimberlite pipes are found only in the oldest and most stable continental regions, far from zones in movement where the rigid plates forming the earth’s crust move apart, disintegrate in contact with each other or collide. Below these undisturbed continental shields the crust is thicker and its weight creates a pressure that is strong enough to form diamonds in the upper part of the mantle, in the area from which magma erupted and then generated kimberlite.
The smallest diamonds, microcrystals of less than half a millimetre, were not taken into consideration for a long period of time because they were difficult to extract; they were therefore regarded as being useless. In recent years, microdiamonds have roused both industrial and scientific interest. For industry, they are perfect abrasives: if fastened to a blade they enable it to cut the most resilient materials like butter. For science they are an enigma: geologists have long understood that they were not formed together with big diamonds, with which they frequently cohabit; their origin is however still under debate. The variety in shape of microdiamonds can be used by geologists as an indicator of the quality and richness of diamond deposits. Microdiamonds have however been discovered in unusual places. They have been found in some continental rocks in Norway for example which have never been exposed to the temperature and pressure needed to form diamonds. Space is however the most interesting place where they have been found. As well as existing in some meteorites, microdiamonds have also been observed in the thick molecular clouds of interstellar space where they form as a result of complex chemical reactions. There are an almost infinite number of possible microdiamond shapes, even if every deposit has a predominant form. Rounded crystals can be found which are probably the result of erosion and deformation. Others are perfect octahedrons, and sometimes even clustered crystals are found, which for geologists are evidence of the abundance of carbon at the moment when they formed. When observed under an electronic microscope, microdiamonds reveal a fascinating complexity. Despite their density, the surface of these crystals is hydrofuge (waterproof) so they can easily float.
Colour is a very important element which determines the beauty and value of diamonds. Natural crystals are almost always of a yellow colour that can vary from very faint to an extremely intense colour. The word fancy colours refers to a diamond with a natural colour, intense enough to be considered a characteristic feature of its appearance. Faint gradations of yellow, grey, brown are considered to be less desirable and diminish the beauty and value of diamonds, whereas intense shades of yellow, orange, brown, grey, blue, pink and red increase the pleasing appearance of the gem and its value. The aspect of fancy diamonds cannot be compared to that of other natural gems; the beauty, brilliance and dispersion of a well cut diamond combined with its spectrum colours enhance its already magnificent aspect. Natural coloured diamonds owe their colour to impurities and imperfections that occurred in the lattice during the crystal’s growth. The most common impurities are Nitrogen and Boron. Nitrogen, for example, in the place of carbon in isolated atoms, or as compact aggregations of atoms is one of the causes of the varying shades of yellow–yellowish orange that can be seen in natural diamonds. Boron, on the other hand, is responsible for the blue colour. Brown colouring is due to structural imperfections in the lattice, whereas a green colour stems from natural irradiations caused by uranium and thorium minerals, located close to the stone.
According to certain characteristics, diamonds are conventionally divided into two types: I and II. - Type I is further divided into Ia and Ib and Type II into IIa and IIb.
The difference depends above all on the presence of differing extraneous atoms in the crystal lattice and is reflected in the absorption of ultraviolet and infrared, in the transmission of light, in electric and thermal conductivity, in the crystal appearance and in the cleavage. All diamonds belonging to type I contain nitrogen. In type Ia the nitrogen–carbon ratio is variable: when it is about 1:1,000,000 stones have a yellow colour, but they become green if the concentration is higher. When the nitrogen-carbon ratio is around 1:1,000, nitrogen atoms aggregate in small clusters called platelets; in this case, absorption no longer occurs in the blue band, but in the ultraviolet one, invisible to our eyes, and the diamonds belonging to this group seem colourless. Type Ib consists of few diamonds with a high percentage of nitrogen which is finely spread in the crystal lattice; stones belonging to type Ib are also paramagnetic, i.e. when set in a magnetic field they have a magnetic strength equal to the field itself. Diamonds of type II have little or no nitrogen. Despite this characteristic, for reasons that are still not clear, they nonetheless manifest a strong absorption in the blue band. Diamonds belonging to type IIa are usually brown and only very rarely colourless. Diamonds of type IIb are crystals which contain traces of boron and, even if only in exceptional cases, present a blue colour, that is sometimes very beautiful: as well as their blue colour, these crystals have another characteristic, that of being semiconductors. Lastly, for diamonds found in meteorites, Type III was created which corresponds to lonsdaleite, in honour of the famous English crystallographer Kathleen Lonsdale, a name which was proposed by Frondel and Marvin, the two men who discovered this mineral in the meteorite that fell near the huge crater in Canyon Diablo, Arizona. It is however very difficult to find diamonds in nature that correspond exactly to one of the types described above; generally the characteristics of one type prevail over the others. This mixture of different lattices is also one cause for the anomalous double refraction that is often seen in diamonds belonging to type I, whereas it is rarely observed in diamonds of type II, which have almost no anomalies in the lattice.
The uniqueness of every diamond requires individual analysis based on numerous grading elements. The fundamental international standards and criteria used to evaluate diamonds are the so-called 4C’s:
Carat – Colour - Clarity - Cut
8.1 - Carat The weight of all gemstones, and hence also of diamonds, is expressed in metric carats. The word carat originated as a natural unit of weight: the seed of the carob tree, characterized by a surprisingly standard weight. Diamonds were traditionally weighed with these seeds until the system was standardized and one carat was fixed at 200 mg, or 1/5 of a gram. Small and tiny diamonds are measured in points, namely in cents per carat. The carat weight of a diamond progressively influences the price since larger stones are comparatively rare. However a small stone of the highest quality may be much more expensive than a larger diamond of lower quality.
8.2 - Colour Individual colours were classified and defined a long time ago. North America was the pioneer in this field, probably because of its high import quota of diamonds for ornaments, which today amounts to 75 % of production worldwide. Colour definitions were established for the first time in the States with a codification that is still effective today at international level and was adopted by different European countries – Germany, the United Kingdom, Switzerland and Scandinavia. The development of electronic devices to measure colour has led to an integration of the original definitions of colour with number or letter systems but not to its substitution, so that even today, the original definitions ( the so called old terms) are still valid. In the set of colours ranging from colourless to yellow, the colour nuances according to old terms are:
River - Top Wesselton - Wesselton
Top Crystal – Crystal - Top Cape
Light Yellow - Yellow.
Most of these definitions, which have survived until today, refer to the diamonds’ places of origin: they are in fact the names of the old diamond mines. So ,for example, the word River has been handed down to refer to diamonds coming from rivers or alluvial deposits. These stones have a better colour than diamonds originating in pipes. The Wesselton Mine produced diamonds mainly of a higher quality than the slightly yellowish gemstones of nearby mines, therefore Wesselton has become the definition for colourless diamonds. The term Crystal may be derived from the word crystal to indicate the faint saturation of yellow. The word Cape derives from the Cape of Good Hope, a coastal land today belonging to the Union of South Africa. Since stones with this origin are on average more intensely yellow than Indian and Brazilian diamonds, diamonds with a stronger yellow saturation were called Cape. Today the most widely used colour scale is the American Color Scale of the Gemological Institute of America (GIA). It is a letter scale ranging from letter D, the best colourless diamond, down to letter Z, diamonds with intense colours.
8.3 - Clarity Very few diamonds can be considered to have no inclusions at all. Inclusions are small natural impurities: minerals, or cleavages embedded in the diamond which are however almost always practically invisible. Crucial for clarity grading is a lens capable of making 10 magnifications. If a diamond has no imperfections after 10 magnifications, it is classified as internally flawless and , therefore, represents the highest grade of clarity. The words used in the clarity scale show how small the differences related to this classification element are.
8.4 - Cut After this presentation of diamonds as minerals, the processes to which diamonds are subjected in order to be used as jewels and become brilliants should be briefly described.
Ideal brilliant cut - In general the term brilliant refers to a diamond cut on every side with many facets.
The phases of working or cutting diamonds are the following: design, flaking, roughing-out, planning the position of facets and lastly polishing. The processing of diamonds has changed very little over the centuries from the time when Indian cutters discovered that by rubbing one stone against another, both opaque surfaces became shiny. They had discovered the fundamental principles of the profession: because of their hardness, diamonds could thus be polished to achieve a unique splendour; because of their capacity to deviate i.e. reflect light, they could be modelled so as to irradiate an intense brilliance and fire. A modern master cutter analyses a rough diamond for days or even weeks, even creating a model in plastic or lead to find the grain in the crystal and decide how to eliminate or minimise the inclusions which may reduce the brilliance of the gem and hence decrease its value. During the cutting and polishing phase, a diamond may lose more than half of its weight and size, but thanks to its radiant beauty, a smaller jewel that has been perfectly worked may far outshine a larger gem. The most common cut is the brilliant cut, i.e. rounded with octagonal shaped crystals, and at least 57 facets, of which at least 32, plus the table, are on the crown and at least 24 on the pavilion. The brilliant cut had already been developed towards the end of the seventeenth century by the Venetian Peruzzi: this cut, with its 57 facets, was considered more beautiful than any of the previous ones, so that even today it is still the most widely used cut. The classic brilliant cut is even today the starting point for new cuts and shapes. The quality of the cut always derives from the correct proportions given to these facets. Only when they are obtained at a correct angle can they give diamonds their unique beauty and permit them to radiate their fire.
To the non-expert eye, one of the numerous stones such as hyaline quartz, beryl, cassiterite, colourless corundum, topaz and colourless zircon may seem to be a diamond. There are even glass imitations on the market, the so called strass. Furthermore there are several materials that are used as substitutions for diamonds: the synthetic rutile, the strontium titanate, the courless corubin, the colourless synthetic spinel. Among the latest rival materials there are YAG (yttrium aluminate), GGG (gadoliniuni gallium gamet) and the Cubic Zirconia. The newest material of great interest is the synthetic Moissanite. Pairs of diamonds can also be seen (diamond crown, pavilion of colourless corundum, hyaline quartz or glass paste). A pair of synthetic spinel and strontium titanate also exists. Natural diamonds irradiated in various ways to achieve an improvement in colour can also be found on sale (Nassau 1989).
Comparative characteristics of diamond and colourless, synthetic stones
In 1970, the General Electric Company produced the first gem–quality synthetic diamonds. The characteristics of these diamonds and of the subsequent productions were carefully studied. (Koivula and Fryer, 1984). In 1990 Sumitomo Electric Industries produced very large single diamond crystals.
Enhanced diamonds are natural diamonds subjected to a particular treatment performed only in two laboratories in the world, in New-York and Tel Aviv. It is a protocol of different treatments, excluding the use of radiation, which are not always performed together, studied specifically for each individual diamond with the purpose of improving its appearance and enhancing quality (intensification of colour with the Gepol method, localized laser, filling of cleavages and cutting). The availability of diamonds exposed to this treatment ranges from ct. 0.90 to more than ct. 2.0: the usage of a lower carat would not be convenient. The stones’ clarity ranges from VS2 to SI, colour from I to J, but yellow and brown diamonds are also available. It is a new and interesting product difficult to find on the market, which offers an interesting opportunity to satisfy customers who desire an impressive natural diamond but are not willing to make a huge investment.
The production and trading of diamonds is managed and controlled for 80 % by a company known as Trade Diamond Corporation, or De Beers Consolidated Mines Limited. These names indicate only a part of an enterprise composed by interdependent, London-based subsidiaries specialized in the production, in the sale and marketing of diamonds. In the field they are called Diamond Syndicate or briefly Syndicate.
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Last modified: Venerdì 20 gennaio 2023