Chapter Five:
The Natural Diamond, its Creation, Extraction, Impurities and Classification
5.1 Diamond Geology
5.2 The Natural Diamond – Its Creation and Bursting Forth
5.3 Natural Diamond Mining Methods
5.4 Diamond Producing Countries
5.5 Internal Flaws in Diamonds
5.6 Types of Diamond
5.7 Diamond Morphology
5.1 Diamond Geology
Over the years, theories have tried to explain how a diamond is “born” deep inside the Earth, but there have always been some unanswered questions. The most important riddle to be solved, was how did carbon reach hundreds of kilometers down into the Earth’s core. Moreover, some years ago, a diamond mine was discovered in Argyle, Australia. This mine and the Australian “kimberlite” takes a different form than in any previously known mine.
Technological advances made over recent years, have made it possible to run tests establishing where a diamond comes from. The latest conclusions on this subject are briefly summarized here below.
5.2 The Natural Diamond – Its Creation and How it Bursts Forth
5.2.1 How the Diamond is Formed
In nature, the diamond is formed within two different types of rocks: Eclogitic rock and Peridotitic rock. According to which source they come from, they are called E or P diamonds appropriately.
A layer of Eclogitic rock is formed by the movement of tectonic plates, which causes the rise of a layer of basalt rock from beneath the ocean floor (the Earth’s crust), along with carbonate and hydro-carbonate sedimentary rocks (limestone and other types), to beneath the Earth’s solid mantle (at a depth of 100 to 250 kilometers).
Eclogite is a rock with a coarse nuclear structure comprising aggregates of red garnet (Almandine Pyroqe) and green pyroxene, along with tiny quantities of rutiles, kyanites and corundums. In this environment, diamonds both grow and absorb impurities.
22 different types of diamond inclusions are known, six of which are common to all types of diamond. 11 are extremely rare and the others can be used to identify the diamond’s origin.
Peridotite is a very common rock in the Earth’s mantle. Some 4,500 million years ago, due to geological activity, a layer of this rock sank to a depth of 100 – 250 kilometers. At these depths, diamonds are formed.
Peridotite is a mineral containing olivine, along with additional mafic minerals rich in magnesium and iron, pyroaxenes and garnets. Diamonds in peridotite are found in garnet bearing harzburgite and the diamonds are formed from homogenous materials. Similar to the formation of diamonds in eclogite, these diamonds are also formed under the conditions of immense pressure and temperature at these depths.
At depths of 100 – 250 kilometers beneath the Earth’s surface, the pressure reaches 45,000 – 60,000 atmospheres (45 – 60 Kb) and the temperatures range between 1,200°C and 1,600°C. Under these pressure and temperature conditions, the structure of the carbon in both these types of rocks changes into diamond. Under these conditions of pressure and temperature, diamonds remain there in a state of balance for 1,200 – 3,500 millions of years.
5.2.2 How Did Carbon Reach Earth?
Like all other atoms, carbon is formed from the initial hydrogen and helium, which formed the stars. Every few tens of millions of years, a star explodes. The thermo-nuclear heat produced at the core of the star when it explodes, causes the formation of materials with large atomic weights, such as carbon and all the metals.
These materials create vast clouds of dust, from which planets are formed, including our own planet Earth. Therefore, the diamonds on our Earth originally came from the stars and that is also true for us.
5.2.3 How Do The Diamonds Reach the Earth’s Surface?
For some reason, two different materials, kimberlite and lamproite have burst up from great depths (deeper than 250 kilometers) in different places and at different times, in the form of an extremely hot fluid under very high pressure. On their way up to the surface, they pass through layers of eclogite and peridotite and they also absorb xenoliths as they rise. Xenoliths are lumps of material, which can be the size of a crystal or as large as a boulder. Diamonds form inside the xenoliths and are carried to the Earth’s surface as the xenoliths move upward. It is now understood that diamonds are not formed in the kimberlite or the lamproites as was thought until relatively recently and instead, they are formed long before the upward eruption begins. The kimberlites and lamproites erupted out to the surface between 100 – 1,200 million years ago.
5.2.4 Eruption Out to the Surface
We assume that chains of cracks from a depth of 200 kilometers, which reach almost up to the Earth’s surface were created for some reason, Usually, the cracks, called dykes, are no more than one meter wide, but in rare instances, they can reach a diameter of 10 meters. The kimberlite forces its way through the cracks and creates a volcanic pipe, which maintains its dimensions until reaching about two kilometers beneath the Earth’s surface. During the last two kilometers of climb, it bursts out in a funnel shape, with an ever-growing diameter as it reaches closer to the surface.
This pipe is similar in shape to a giant carrot.
5.2.5 The Kimberlite Pipe
The kimberlite pipe is divided up into three areas (Diagram Number 5 - 1).
A. The root – This section stretches from deep inside the Earth to a distance of about 2 – 2.5 kilometers from the Earth’s surface. Root dimensions are tiny and the speed of kimberlite travel within the root was relatively slow: 10 – 30 kilometers an hour.
B. The area of the funnel, called the diatreme, comprises sedimentary rock and it is the most important area for diamond extraction. With a depth of between 1 to 2 kilometers, it continues up to about 300 meters below the Earth’s surface. This area also contains material, which has collapsed from the funnel walls in which the kimberlite breccia have formed. Rate of flow in the funnel was very rapid, because of the immense eruptions releasing vast quantities of dissolved gases, principally carbon dioxide and water in the kimberlite. Under incredible pressure within the kimberlite, they are released at lower pressure when reaching close to the surface.
As the result of these explosions, the upward speed of the kimberlite rises to reach several hundred kilometers an hour. The powerful flow of kimberlite expands and begins to shape the cone which continues to expand as it reaches closer to the surface. Rapid expansion of the kimberlite also causes it to cool down rapidly and therefore, it did not flow out as a very hot liquid as is true in the case of lava in an volcanic eruption. Instead, it takes the form of a hot solid, broken down into blocks of different sizes.
Under these pressure and temperature conditions, the diamond can continue to exist, without undergoing thermodynamic changes (Diagram Number 5 – 2). This is the process, which allows the diamonds to remain as they were, during the rapid transition from very high pressure and temperature to the new set of conditions across the Earth’s surface.
C. The crater zone – In the last 300 meters before reaching the surface, the material rises to take the shape of a volcanic cone, reaching a height of several hundred meters, but without the liquid lava core. Instead, it contains hot, dry materials in the form of rocks and ash called pyroclastics. (pyro = fire; clastas = fragments) which have been pushed upwards. Over millions of years, eroded materials build up inside these cones at the rate of about 1 meter every 30,000 years. Over time, the hills become craters at the mouth of the kimberlite pipe and can reach a diameter, which is tens of kilometers wide.
Diagram Number 5 – 1: The Birth of a Kimberlite Pipe
Legend:
(1) Kimberlite bursts up through cracks in the rock.
(2) The cracks reach the Earth’s surface.
(3) The cone is formed
(4) Eruption ends.
(5) The cone has disappeared almost completely
(6) The pipe becomes a crater.
Diagram Number 5 - 2: Xenolith containing a diamond
In South Africa, there are some 3,000 kimberlite pipes (100 in North America), but less than a thousand of these pipes are commercially viable and only 50% - 60% are currently exploited. Craters have areas ranging between 12 – 75 hectares and yield one carat per 5 tons of kimberlite. Kimberlite appears as accumulations of between 6 and 40 dykes, one beside the other.
Kimberlite eruption areas can be classified into three different types:
1. Areas in which the kimberlite did not pass through very deep layers of eclogite, located beneath the peridotite layer.
This type of pipe mainly produces P type diamonds and a small number also produce E type diamonds.
2. Pipes in which the kimberlite did not pass through the peridotite layer and produces E type diamonds.
3. Pipes in which the kimberlite reaching the surface contains no diamonds whatsoever. Apparently, this is because flow speeds were low and following the creation of the appropriate conditions, the diamonds are transformed into graphite and gas, such as in Beni Bouchera in Morocco.
4. Lamproite pipes were discovered in 1979 at a distance of some 400 kilometers from Argyle, Australia. (Argyle Diamonds 1 and Ellendale). This pipe is not vertical and
evidently flowed sideways from a large number of channels simultaneously. Some of the channels went through a peridotite layer and others flowed through an eclogite layer. Therefore, the diamonds extracted at Argyle are of both types – E and P. The lamproites contain significant quantities of Potassium – about 6% - 8% K2O. The percentage of CO2 in this material is very low, relative to the quantities found in kimberlite.
The lamproite flowed just like kimberlite, but when it reached just beneath the surface, the pipe took on the shape of a champagne glass – a sudden widening – and it does not have the carrot shape, which is the classic shape for a kimberlite eruption.
Most of the kimberlite and the lamproites were formed during the last 200 million years. The oldest were formed some 1,600 million – 2,600 million years ago and as early as one million years ago in Wyoming and the Antarctic.
5.3. Natural Diamond Mining Methods
Diamonds are mined from within the former “pipes”, along ancient river estuaries and in the seas, into which ancient rivers flowed, carrying the alluvial deposits washed away after the eruptions. Mining methods are as appropriate for each type of deposit.
5.3.1 The Different Mining Methods
Open Cast Mining (Diagram Number 5 – 3 and 5 – 4)
This mining method has been used since diamonds were first discovered. Explosives are used to blow apart the kimberlite rock, which is transported away from the hole created by conveyor belt, or it is loaded into trucks (the method commonly used in Russia). Open cast mining will normally reach to depths of some 200 – 220 meters.
Subterranean Mining (Diagram Number 5 – 5)
This is a more complex mining method, because the work is done beneath the perforated rock, which is structurally, relatively weak and is therefore problematic. To enable easy access to the mine and overcome ventilation problems, two shafts are sunk to depths of some 2 kilometers, at a distance of about 300 meters from the pipe, or in other words, into the rock surrounding the pipe. At different depths inside these shafts, horizontal shafts are dug out at a distance of some 200 meters from each other, which reach into the pipe. The horizontal shafts are equipped with rail tracks, to carry bulk cars, which remove the ore (Diagram Number 5 – 5) Vertical shafts are equipped with very fast elevators, each with the capacity to lift 20 tons of ore to the surface. This method of mining is used after full exploitation of the open cast mining process.
Alluvial Mining (Diagrams Numbered 5 – 6, 6 – 7 and 6 – 8)
This method is used to mine ancient alluvial deposits, such as along ancient rivers and shorelines. Heavy moving equipment is used to remove millions of tons of sand, which covers the ancient river course to a depth of 8 to 15 meters. All the ancient deposits, including gravel, are transported for separation and sorting at a nearby plantf.
All the material found between rocks at the ancient river bottom is also removed for screening. Diamond production in this type of mine, reaches 1 carat for every 25 tons of material.
Diagram Number 5 – 3: Opencast Mining and Separation Plant
Diagram Number 5- 4: Jwaneng Mine in Botswana – 200 meters deep
Diagram Number 5 – 5: Subterranean Mining
Diagram Number 5 – 6: Panning for diamonds along a river bank.
Diagram Number 5 – 7: Clearing an ancient river course
Diagram Number 5 – 8: Clearing the material between the rocks on the river bottom
Diagram Number 5 – 9: Offshore mining vessel
Offshore Mining (Diagram Number 5 – 9)
Alluvial deposits flowed out to sea carried by the ancient rivers. In 1961, a barge was equipped for dredging at distances varying between 2 and 5 kilometers from the shoreline.
The barge vacuumed up all the sand between the rocks on the seabed and then filtered out and separated the diamonds from the sand (Diagram Number 5 – 7). Ancient alluvial deposits are known for the high quality of their diamonds and very high yields of gem stones. Naturally, diamonds that have survived the silt, erosion and battering by sand and rocks carried by the currents and waves, are the hardest diamonds.
Separating the Diamonds
As previously mentioned, the number of diamonds per volume of mined ore, varies between 1:25,000,000 and 1:125,000.000. (from one to twenty five million, to one to one hundred and twenty five million). In other words, the diamonds must be extracted from enormous quantities of ore, without damaging them, because the bigger they are, the more they are worth.
The most commonly used method to separate the diamonds has a number of stages:
1. The rock in which the diamonds are embedded is ground and pulverized to release the diamonds.
2. Separation by specific weight.
The first separation stage exploits the diamonds’ high specific weight (about 3.5 grams per cc). The ore is loaded into a cone containing a mixture of iron and silicon dissolved in water.
This solution has a specific weight of between 2.8 – 3.1 grams per cc and therefore, materials with a lower specific weight will float and those with a higher specific weight, including diamonds, will sink. The concentrations achieved by this method vary between 1:30 and 1:100.
To control the separation efficiency at this stage, metal shards are manufactured in typical diamond shapes and sizes. The metal is an alloy of zinc and aluminum, with a specific weight of 3.5 (the same as diamonds). The alloy is irradiated in an atomic reactor, where the zinc becomes a, radioactive material giving off radiation, which can be tracked by radiation detectors, which display the percentage of metal particles lost in the process. This information also indicates the percentage of diamonds lost in the process.
3. Grease Table Separation
It is well known that diamonds are hydrophobic – they are water repellent. Therefore, the ore is mixed with water and is then allowed to run down a plate or belt covered in grease or heavy duty lubrication oil. Particles of different materials wet with water, will slide over the greasy surface, but diamonds, which are water repellent, remain dry and they adhere to the grease (Diagram 5 – 10).
Diamonds from sources closer to the oceans, are often coated in a layer of salt, which is water soluble. The addition of specific chemicals results in the diamonds receiving a coating of non-wettable material, which does adhere to the grease. However, this coating process can also sometimes act on slag and many slag particles also adhere
to the grease.
Diamonds are separated from the grease by heating and filtering the grease, following which the diamonds are sorted by hand. However, this sorting process does present difficulties such as:
A. Large quantities of slag adhere to the grease as well as diamonds
B. It is difficult to sort out very small diamonds.
Diagram Number 5 – 10: Belt covered in grease, with adhering diamonds
5.3.2 Additional Methods for Diamond Separation
A number of additional methods are also available for diamond separation:
1. Diamonds fluoresce under x-ray illumination. A stream of ore is passed beneath a beam of x-rays and as the diamonds pass beneath the beam, they fluoresce. A photoelectric cell detects the fluorescence and opens an appropriate gate, which transfers the diamond to a different track.
2. Most diamonds do not conduct electricity. This is a property which can be utilized for electrostatic separation and this method is used efficiently to discover:
A. Very small diamonds
B. To process diamonds extracted from alluvial deposits. Because these diamonds are coated with a layer of salt, which dissolves in water it is difficult to separate these diamonds over a grease table.
3. Even after electrostatic separation, some of the remaining minerals have properties similar to diamonds, or in other words, their specific weight is high and they are poor conductors of electricity. To remove these minerals, a thin layer of ore flows into a funnel containing hydrochloric acid and after staying in the funnel for a short time, it then flows into a funnel containing cold water. Most of the liquid is emptied out of this funnel and the residual sludge contains the liquids with specific weights higher than diamond. The funnel is shaken gently and the diamonds begin to float. They are then skimmed off the surface. This process is repeated until no more diamonds float to the surface.
4. Magnetic minerals can be collected using an electro-magnet.
5. At temperatures of about 400°C, most minerals dissolve in soda-caustic, while diamonds are unaffected by this material.
5.4 Diamond Producing Countries
The number of diamonds mined around the world rises from year to year and every so often, a new diamond mine is discovered.
The following is a list of the countries currently extracting diamonds, listed by continent:
5.4.1 Africa
Angola
This is one the countries richest in diamonds on the African continent. Over 50 diamond fields have been discovered in Angola, some rich and some small. 80% of
the diamonds extracted are ornamental stones and total production reaches some 1.5 million carats annually. In view of the continuing civil war in Angola the diamond mines have not been exploited to their full potential. Currently, diamond production is 77% state owned and the remaining percentage is owned by D.T.I. After the war ends, it is reasonable to assume that production will rise by hundreds of percentage points. As early as 1974, Angola produced some 2.5 million carats.
Botswana
Botswana’s most famous diamond mines are Orapa, Letlhakane and Jwaneng, which is the world’s richest mine. Botswana produces some 16 million carats annually. Production began in 1982 as a joint venture with De Beers. Botswana is an independent country with very strong ties to South Africa.
Ghana
Most diamond mining in Ghana is alluvial mining along river banks. Production reaches some 140,000 carats a year. Some production activities are unsupervised. Most of the diamonds extracted are small and only 25% are ornamental. Political instability has an adverse effect on production.
Guinea
130,000 carats annually. The Aredor mine produces large, high-quality stones.
South Africa
South African diamond mining takes three different forms: Pipes, alluvial and offshore.
The most famous pipes are: Kimberly, Premier and Jagersfontein. In total, there are some 20 diamond mines in South Africa, which is one of the six largest producers, extracting some 9 million carats annually. Each pipe produces a different colored diamond. The Premier mine produces blue diamonds.
Zaire (Miba)
Alluvial mining and most of the diamonds extracted are small. Overall production reaches some 20 million carats a year. 80% of the production is industrial diamonds, some of which are bort. Zaire is ranked second out of the six largest producers.
Ivory Coast
Alluvial mining and most of the diamonds produced are small, high quality gemstones.
Tanzania
Due to technical and economic problems, this country, which is rich in diamonds, produces no more than 150,000 carats annually.
Liberia
Alluvial mining along river banks produces very low quality diamonds.
Central Africa
Alluvial mining; some of the stones produced are colored. Diamonds have good shape. Produces some 600,000 carats per year.
Namibia
Namibia is very rich in diamonds (see: Chapter 2 – History). Recently, Namibia won its independence and it has begun taking an important place among the major producers. Currently, Namibia produces some 1,000,000 carats annually.
Sierra Leone
Produces some 600,000 carats a year. Organizational difficulties make production difficult.
5.4.2 America
USA
The quantity of diamonds produced in the USA is negligible. The most important diamond bearing strata are in the Sierra Nevada.
Brazil
Brazil is one of the oldest diamond producing countries. Very large diamonds have been discovered in Brazil, with the biggest reaching 762 carats.
At Mina Gerious, 140,000 carats are produced annually and these diamonds are high quality, they come in a range of colors and reach sizes up to 8 carats.
At Mato Grosso, elongated diamonds can be found. Most of the diamonds extracted are colored, small stones, but “carbonado” diamonds weighing from 30 – 110 carats are also extracted at this mine.
Rio produces small, very high clarity diamonds, as well as carbonado Balas Gorbonoudo or Canarieiras diamonds, known for their extreme hardness.
Guyana
Negligible total production of some 100,000 carats per annum. The State Government is currently seeking investors to develop new fields, which undoubtedly exist.
Venezuela
Most Venezuelan diamonds are small and high quality. In recent times this country’s production levels have fallen.
Canada
Miniscule level of production of tiny diamonds. Recently, some of the diamonds produced have reached 3 – 4 mm. There are high hopes of finding diamond fields similar to those in Russia.
5.4.3 Asia
Indonesia
Alluvial mining. The diamonds produced have excellent color and quality and fancy grade color. In Borneo, large diamonds have been found. Indonesia produces about 120,000 carats annually at this stage.
India
The oldest diamond producer in the world, from a substantial number of fields and the diamonds are of good quality. Currently, there are five active mining areas.
China
China has also been known as a source of diamonds for hundreds of years. Most of the stones are very small and in the past, they were used for glass cutting. There is a good chance that there are rich diamond strata in China. Recently, a number of the diamonds found have reached tens of carats or even over 100 carats, with good quality. China produces about 200,000 carats a year and has significantly large reserves.
Russia
The first Russian diamonds were found in the Ural Mountains. Russia’s became famous as a diamond producing country through its diamond mines in Siberia, which were discovered in 1954. The Siberian mines are located in the far north, where the ground is frozen to a depth of hundreds of meters and in the east. In terms of industrial diamond production, Russia is ranked fourth out of the top six producers and in terms of ornamental diamonds, it is placed behind South Africa. About 16 diamond producing fields have been found in Siberia.
Russian diamond production constitutes 25% of world production and the country has very substantial diamond field reserves. Russia produces some 12 million carats a year.
5.4.4 Australia
Australia has been known as diamond producer since 1884. Until some years ago, diamond production from well established fields was relatively low and unimportant in world terms. This changed drastically with the discovery of the Argyle field, which contains vast numbers of diamonds, 80% of which are industrial diamonds.
Australia produces about 37 million carats a year and is the worlds largest producer. Australian ornamental diamonds are typically pink – which is considered very rare. Several new Australian mines are currently in the development stages and in the future, they will be able to produce very high quality ornamental diamonds.
See: Diagram 5 – 11: Map of the Diamond Producing and Processing Countries.
5.5 Internal Flaws in Diamonds
Most of the flaws in natural diamonds are either man made, or they are created during the crystal growth of the diamond. These flaws have either an adverse effect on a diamond’s value or increase its value. Flaws can be divided into two main groups, depending on flaw size.
5.5.1. Diamond Inclusions
The diamond crystallizes around a foreign body or even around diamond in solution. After completing crystalline growth, the nucleus remains within the diamond. Inclusions can be of various types and comprise a range of different minerals, such as Sulfides, graphite, spinel, silica (even in the form of a dark cloud), alumina, potassium; or they
can even be gem stones in different colors, from black to white, including all the colors in between. Some large inclusions have a different coefficient of expansion from the surrounding diamond and during the processing of the diamond (cutting or polishing), high tensile strain levels develop, which can cause the diamond to explode.
Strain around inclusions are visible under fluoroscopy. The picture seen around the inclusion site, can have all the colors of the rainbow. That is particularly true in the case of zirconium inclusions, which can appear in a range of colors, such as: White, yellow or brown. The inclusion can also be a diamond “embryo”, arranged in a form which is not identical to the surrounding diamond structure. In the professional jargon, these inclusions are called a “pique” or “piques” in the plural.
Identification of the different types of inclusion is performed by examining their spectra using a method which measures molecular vibrations (created by a laser beam directed at the inclusion). Each material produces different lines in different parts of the spectrum and from those lines, it is possible to deduce the nature of the inclusion and even the original source of the diamond.
Pique inclusions can be removed, using a laser beam to drill into the diamond to reach the inclusion itself, which is removed chemically. If the inclusion reaches up to the diamond’s surface, it is easy to remove. Laser drilling will always leave a slightly conical “pipe”, with a diameter of some hundredths of a millimeter and a depth of 8 – 10 mm. It is not the usual practice to fill the pipe with a transparent material.
If an inclusion is not graphite and is a red or another color, it is an oxide inclusion, around which the diamond crystal formed, which can also be removed using this method.
In recent years, a material based on lead salts has been developed, which can penetrate into the drilled hole, or existing holes and plug the hole in a way that is invisible to the human eye. The fill material’s light refraction coefficient is very close to
the coefficient for diamond. X-ray screening can reveal these plugs.
Minerals and graphite are often found in diamonds, particularly diamonds from India. Pink, gem stone inclusions are often found in diamonds from Brazil. Sulfides and orange or green gem stones appear regularly in diamonds from South African mines. Usually, inclusions are part of the mother material in which the diamond crystals grow.
5.5.2. Impurities
A good, gem quality diamond contains some 0.1% impurities. To date, some 58 foreign elements have been identified in diamonds and 44 of these elements have been checked quantitatively.
Other impurities in natural diamonds, apart from nitrogen include: Aluminum, with up to 100 ppm (100 parts in a million); in man-made diamonds, the quantity of aluminum can reach 150 ppm. Silicon, magnesium, iron and calcium appear in diamonds in quantities that reach about 10 parts in a million. Copper, barium, strontium, sodium, silver, titanium, chrome and lead also appear, but in very tiny quantities. Another important impurity is boron, which can appear in natural diamonds with a concentration of 0.3 ppm (in man-made diamonds, up to 270 ppm) and it suffuses the diamonds with a blue color.
These materials were discovered by evaporating diamond dust within an electrical arc. Every element has a characteristically colored vapor and the color is checked using a spectroscope. Impurities extant, but distributed diffusely as individual atoms,
affect both electrical and optical properties. The diamond as a gem stone can lose its “life”, if the impurities have a color absorbing effect (see: Chapter 3 – The Atom and Energy Levels). Impurities present in larger concentrations, increase the diamond’ s resistance to cleaving.
To a great degree, nitrogen and boron impurities determine diamond color and value; therefore they are of great interest. Yellow diamonds are a lot less expensive than white diamonds, while blue diamonds are more expensive that white diamonds. That is why we will spend some more time discussing the nitrogen and boron impurities in diamonds. Both nitrogen and boron replace a carbon atom in the diamond’s crystalline structure.
1) Nitrogen in Diamonds
As previously mentioned, an atom of nitrogen is close to carbon in terms of its atomic weight, but has an additional electron in its outer shell. Nitrogen is willing to donate this electron and is therefore called a donor. The lonely electron will cause the creation of a new level of energy in the forbidden band or energy gap (see: Chapter 3 – The Atom and Energy Levels).
As they grow, almost all diamonds are polluted by nitrogen. Some diamonds have very low levels of nitrogen impurities, just a few nitrogen particles per million carbon particles (a typical yellow diamond contains one nitrogen atom for each 100,000 atoms of carbon and the carbon appears as an individual, interstitial – penetrating atom. Sometimes the concentration of nitrogen atoms is very high and can reach 0.3% of the
carbon atoms.
Nitrogen in the diamond can appear as an individual atom or as a group of atoms with 2, 3 or 4 atoms of nitrogen distributed in the crystal (Table Number 5 – 1). One theory suggests that the nitrogen was included as an individual atom, as early as during the crystallization process and during the later period, when the diamond was still at a high temperature in the depths of the Earth, the nitrogen atom wandered slowly until settling down in different aggregate forms.
In diamonds containing individual nitrogen atoms, the energy required to release the nitrogen’s additional electron from the valence band level (or one of its intermediate levels in the forbidden band for carbon) and move it to the conductivity band (or to a higher intermediate band in the nitrogen) (see: Chapter 3 – The Atom and Energy Levels, Diagrams Numbered 3 – 15 and 3 – 16), is lower than the energy required to release one carbon electron from the valence band to the conductivity band. The energy required to release one nitrogen electron is equivalent to 4.4 eV = 310 nm (4 eV near ultraviolet), compared with 5.4 eV =220 nm (far ultraviolet) required to release a carbon electron. The quantity of energy is inversely related to the wave length (E = hc/λ). The blue – violet color in the light, which is at an energy of 2.5 – 3 eV, cannot release the electron from the nitrogen and therefore, it will not be absorbed up by the “donors” and the diamond shall remain a yellow color.
A higher quantity of nitrogen will cause a change in the donors’ energy levels in the forbidden band and the color will tend towards the green or even towards almost
complete black.
If the quantity of nitrogen (individual atoms) in the diamond is less than 0.3%, there will be no absorption in the 310 nm range. When the nitrogen concentration is high, the nitrogen tends to arrange itself in groups of:
2 atoms (pairs) called A Center
3 atoms in the (111) planes called N3 Center
4 atoms of nitrogen called B Center
A Center and B Center absorb light in the UV range and do not have any influence on stone color. An N3 Center absorbs at 415 nm (violet). This absorption reduces the blue component in visible light and the stone will appear to be yellow (complementary color). This is the most common absorption and is caused by N14 atoms of nitrogen, replacing C12 atoms of carbon
Small concentrations of N3 Centers lack any color and can add “fire” to the polished stone by absorbing the ultraviolet light and emitting visible light.
Under special N3 Center conditions, there are absorption bands within the visible spectrum at 453nm, 463 nm and 478 nm. These bands (in the spectrum) are weak, but do influence stone color because of the eye’s sensitivity and the thicker the diamond, the stronger the color will be.
Table Number 5 – 1: Nitrogen Structure in Diamond
|
Crystalline Structure |
Type |
Absorption |
|
|
| 1. __ N __ | |
Ib |
Absorption at λ < 500 nm |
Color
Forming
Centers |
|
| | 2. __ N __ N __ | | |
Ia |
System with head line at 415 nm |
|
|
| | 3. __ N __ N __ + defect | | |
|
System with head line at 503 nm |
|
|
| | 4. __ N __ N __ | | |
|
Absorption at λ < 320 nm |
|
|
5. |
|
Bands
of the stepped type |
|
2) Boron in Diamonds
Boron atoms contain one electron less than carbon atoms and therefore, by replacing a carbon atom by a boron atom, we reach the acceptor’s level in the forbidden zone (as described in Chapter 3 – The Atom and Energy Levels, Diagram Number 3 – 15). This atom can receive an electron from the valence level and as a result, light can be absorbed on its way through the diamond. In accordance with the initial energy level of
this electron in the valence level, different energy absorption levels are possible and the color red will be absorbed because its energy matches the energy required to raise the missing electron to the “acceptors” level. In this circumstance, the result will be a blue color.
Another consequence to the existence of the “acceptors” level, is that a relatively low amount of energy (0.4 eV) is required to raise an electron from the upper end of the valence band to the level of an electron in a boron atom at 0.4 eV above the valence level. This energy level can be achieved at room temperature (using thermal excitation). Furthermore, the rise of the electron causes the creation of “holes” in the valence band. These “holes” can move under the influence of an electric field and can cause a blue diamond to conduct electrical current in the valence range. The creation of the blue color under the influence of boron in diamond is a natural process (as in the case of the blue “Hope” diamond) and also occurs in synthetic diamonds, when boron is added during crystal growth (usually, one boron atom for every one million carbon atoms). The claim is made that aluminum impurities in natural diamonds can also cause the blue color, but when added to synthetic diamonds during the growing process, no color results. Aluminum is also found as an impurity and aluminum also has one electron less than carbon, but it does not create a color because the energy level for the aluminum impurities is higher and the electrons from the valence level cannot reach that energy level. Blue diamonds that can conduct electricity are classified as Type IIb.
In contrast with a diamond containing boron impurities, a blue diamond created during
an irradiation process and in which the color is caused by concentrations of color, does not conduct electricity.
5.6 Types of Diamond
Depending on the nitrogen structure within the diamond, we classify the diamond into two main groups: I and II – and sub-groups: Ia, Ib, IIa, IIb.
Diamonds that contain a sufficient quantity of N nitrogen (above 20 ppm), which can be revealed through an examination of these diamonds optical properties, are called Type I diamonds.
The other diamonds, which contain a smaller percentage of N nitrogen, or which contain no nitrogen atoms are called Type II diamonds.
Type I Diamonds
Type I diamonds are divided into several intermediate groups:
Ib – Type I diamonds in which the N nitrogen atoms are present individually. These diamonds have a strong greenish color.
Ia – All the other diamonds, which can be divided up again, according to the N nitrogen structure:
Type IaA Diamonds
These diamonds contain nitrogen in A centers (two nitrogen atoms as a pair) in the form of small “leaves”. These diamonds lack any color and they are transparent to
visible light.
Type IaB Diamonds
These diamonds contain nitrogen in B centers (four adjacent atoms, so it is believed, together with one vacancy).
Type IaA/B Diamonds
Most naturally occurring diamonds contain A and B centers together and they are called Type IaA/B. Diamonds of this type usually contain small platelets in cube planes. These are surface flaws with sizes ranging from 10 nm to several microns. Diamonds of this type (Type IaA/B) also contain very small quantities of their nitrogen as N3 centers, which grant the diamonds, a faint yellowish color.
In the process which creates color centers (see: Chapter 8 – Color in Gemstones), which give the diamond a yellow color, the principal factor is the nitrogen impurity and in one instance, an aluminum (Al) impurity. Diamond color is a unique phenomena and nothing similar is found in any other mineral (with the exception of silicon carbide, in which nitrogen impurities result in a green color). It is interesting to note that there is little difference between the quantity of nitrogen in a colorless diamond and the amount found in a colored diamond. The conclusion that therefore must be reached is that the color is created in the diamond only when the nitrogen taking part in the creation of color concentrations has a special structure.
Type II Diamonds
Once again, this type of diamonds, is sub-divided into two sub-groups, differentiated from each other by their electrical conductivity properties:
IIa – Diamonds that do not conduct electricity
IIb – Semi-conducting diamonds (110 ohm/cm) at room temperature. The electrical resistance of these diamonds drops as temperature rises and therefore, they can be used as thermometers or as transistors at relatively high temperatures.
Type IIa diamonds are the best conductors of heat (five times better than copper) at room temperature and they conduct heat 25 times better than copper at the temperature of liquid air. These diamonds are colorless and most have the color D.
Type IIb diamonds are manufactured artificially today in accordance with the required electrical conductivity. They appear as blue, grey or deep blue diamonds, or even as black diamonds.
A summary of all these properties is given in the table below (Table 5 – 2).
Diagram 5 – 12: Nitrogen structure in different types of diamonds (100)
A. Nitrogen platelets in diamond at a magnification of X 90,000
B. Type II (100) diamond without nitrogen platelets with dislocation at a
magnification of X 75,000.
C. Nitrogen needles in diamond at a magnification of X 75,000.
Diagram 5 – 12 (continued):
Nitrogen
structure in different types of diamonds (100)
Table 5 – 2: Nitrogen Structure in Diamond
5.6.1 Diamond Types
Often, crystal diamonds appear coated with something like a shell, which comprises sealed layers of diamond, called coats. Coats must be removed to reach the clean inner diamond. Cleaning can be a mechanical process, during which the coats are split apart, but usually, chemical digestion methods are used, or they are burnt away in a gas mixture, which burns the coat, leaving the clean diamond behind.
Another type of diamond is called the “Balas” type, which is usually, a multiple crystal diamond with a ball shape. This type of diamond cannot be processed and will not be used as an ornamental gem. It is used for drilling or the preparation of the polishing channels for diamond polishing wheels (their source is from Rio in Brazil – see Clause 5.4.2.).
Carbonado diamonds are large diamonds, which are aggregates of micron-sized diamond crystals and often, there are gaps between the crystals. Therefore, this type of diamond does not have the same specific weight as regular diamonds. Carbonado diamonds lack any defined shape and their black – grey color is opaque. Usually, carbonado diamonds appear as large clumps weighting dozens of carats. It is not an ornamental diamond. The accepted treatment is to break it down into parts with an appropriate size, which are then “set” into drill bits. Carbonado diamonds are isotropic and have no cleavage planes.
5.7 Diamond Morphology
5.7.1 The Crystalline Structure of Diamonds
The crystalline structure of diamond is the reason why it is so hard. It is also the reason why it shines, takes a certain shape, it provides the cleaving and sawing directions and most of the diamond’s other properties can be explained by its crystalline structure. In nature, most diamonds are found in well defined crystalline form, but as is true for all crystals, they are almost always slightly distorted. This distortion is the reason why it is difficult to find two identical crystals. Crystal shape is of great importance in terms of the diamond’s value. The shape of the diamond determines the percentage of wastage during processing, which can be less than 30% or more than 50%, depending on the shape of the raw diamond.
It is worthwhile reviewing a number different diamond crystal shapes: Octahedron, cube, rectangular and rhombic dodecahedrons. These shapes are of enormous importance, for reasons that shall become clear later on.
5.7.2 The seven fundamental shapes for the diamond crystal.
It has already been mentioned that diamonds crystallize in a cubic system. This system is typified by shapes with equal dimensions, or in other words, ideal cubic forms that are not elongated or flat. If we surround this type of cube with a ball, with a diameter equal to the diagonal of the cube trapped within it, 2 – 3 apexes will touch the ball. Crystallographers describe this cubic system as a system with three axes of symmetry, equal in their length and perpendicular to each other.
Every mineral that crystallizes in a cubic system with a high level of symmetry, can develop characteristic shapes – such as the cube or the octahedron. Alternatively, it can be a combination of two or more of the seven basic shapes given below:
The seven basic shapes are:
A. The cube or hexahedron
B. The octahedron
C. The dodecahedron
D. The tetrahexadron
E. The icositetrahedron
F. The trisoctahedron
G. The hexoctahederon
A. The Cube or Hexahedron – Diagram Number 5 – 13
This crystal shape comprises six square faces perpendicular to each other. Each face cuts one axis of symmetry and is parallel to its opposite face. A simple cube or cube
shape with additional shapes is very rare in ornamental gemstones, but it is common in industrial diamonds.
B. The Octahedron – Diagram Number 5 – 14
This crystal shape has eight triangular faces. Each face cuts all three axes of symmetry, at equal distances from the center. This shape constitutes the “habit” – the shape most common in ornamental diamonds and it is often accompanied by other shapes.
Diagram Number 5 – 13: The Cube
Diagram Number 5 – 14: The Octahedron
C. The Dodecahedron – Diagram Number 5 – 15
(12 faces) This shape comprises 12 rhombus faces, each of which bisects two of the three axes of symmetry at equal distances and is parallel to the third. This shape is less common in ornamental diamonds and is also known as a rhombic octahedron.
D. The Tetrahexadron – Diagram Number 5 – 16
Comprises 24 triangular faces. It is similar to a cube in which the faces have been replaced by four triangular faces. This is an unusual shape for a diamond.
Diagram Number 5 – 15: The Dodecahedron
Diagram Number 5 – 16: The Tetrahexadron
E. The Icositetrahedron - Diagram Number 5 – 17
Comprises 24 trapezoid faces. This shape is similar to the dodecahedron, but each of the dodecahedron faces has been replaced by three square faces. This is also an unusual shape for ornamental diamonds.
F. The Trisoctahedron - Diagram Number 5 – 18
Comprises 24 faces in the shape of an equilateral triangle. Similar to the icositetrahedron, it can be viewed as an octahedron, with each face replaced by three triangular faces. This is a common shape in ornamental diamonds.
Diagram Number 5 – 17: The Icositetrahedron
Diagram Number 5 – 18: The Trisoctahedron
G. Hexoctahedron – Diagram Number 5 – 19
Comprises 48 triangular faces. Each face bisects all three axes of symmetry in a different way. Hexoctahedrons appear in different forms, as appropriate to the position of face bisection with the axes. The hexoctahedron can be viewed as an octahedron, with each face replaced by six triangular faces.
Diagram Number 5 – 19: The Hexoctahedron
5.7.3 The origins of the names for each type of crystal
The names used for to describe the different shapes explained above, appear to be very complicated. To understand them, it is important to realize that they are all created by the combination of a small number of simple roots (originally in ancient Greek).
Hedron = Face
Tris = Three
Tetra = Four
Hexa = Six
Octa = Eight
Dodeca = Twelve
Rhombus = Equilateral parallelogram
Trapeze = Square with two parallel sides
So, for example, a hexahedron has six faces and a tetrahedron has four faces, an octahedron has eight, etc. The tetrahexahedron has 6 x 4, or 24 faces. The word trapeze or hedron describes the shape of the faces and not their number.
5.7.4 Deviation from the ideal crystalline shape
All the shapes described above are ideal shapes. In fact, these ideal shapes are created in nature only very rarely. The reason for this rarity is that there are many factors with an adverse influence on crystal growth in ecological matrices.
Diamond crystals very often appear in the form of an octahedron but with faces that are
rough, distorted or full of perforations. Elongated or flat crystals are very commonly found. The combination of distorted faces and a multi-faced shape such as the tetrahexadron, often creates crystals which are almost round.
5.7.5 Combinations of shapes
In most cases, naturally formed crystals are found to be a combination of two or more of the basic crystal shapes. Usually, diamond crystals tend to be dominantly octahedron shapes, which is sometimes combined with at least one other shape. The most common combinations are the octahedron with the icositetrahedron; or the octahedron with the hexoctahederon. It is important to note that both these combinations create the impression of an octahedron with rounded faces. The combination of the octahedron and the Dodecahedron is shown in Diagram 5 – 20.
Diagram 5 – 20: The Combination of an Octahedron and a Dodecahedron
5.7.6 Signs of Growth on Diamond Crystal Faces
Identifying a diamond one of the most important problems faced by this industry. The identification methods used can be divided into two categories:
A. Identification using flaws on the diamond’s surface
B. Identification using radiation
Often, the diamond polishing process is not completed perfectl