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What is a diamond?

 

Our love of diamonds throughout the centuries has resulted in many myths and legends that have built up around this exquisite mineral. In much of the world’s folklore, tales abound of men who risk their lives to search for them. In these myths, a hoard of diamonds is often guarded by beasts such as dragons, or serpents as in this piece written about Alexander the Great and his men who were exploring the desert of the Scythians, Iranic equestrian tribes who inhabited large areas in the central Eurasian steppes between the 7th Century BC and the 4th Century AD.

"From their summit, the valley floor, gripped in a heavy mist, cannot even be seen. The valley floor is strewn with brilliant diamonds, beyond the reach of human hands. The valley is guarded by serpents and no man can gaze upon them without dying. The serpents’ power remains as long as they live, with death, they lose their power."

It is said that Alexander the Great placed a large mirror within the valley of serpents. When they approached they saw a reflection of themselves, meaning instant death. This gave Alexander and his men the opportunity to retrieve the diamonds from the valley, but none of his men were willing to undertake the task of descending to the floor of the valley.

The Council of Elders advised him to slaughter sheep and throw pieces of their flesh into the depths of the valley. The diamonds which lay on the floor of the valley became attached to the flesh. Hungry eagles devoured the flesh, then bore them up, after which Alexander ordered his men to hunt the birds and retrieve the diamonds.

The world of diamonds ~ an introduction

 

It is known that diamonds were first discovered in India, around 800 BC which until relatively recently was the only global source and the most credible reference to diamonds is known to have also originated from here. “The Lesson of Profit” translated from a Sanskrit manuscript, and dating from the 4th Century BC, was discovered in 1905. It was knows as the Arthasastra. From this manuscript it was acknowledged that from 800 BC the trade in diamonds was a thriving market, as well as being an exquisite mineral being used for personal ornamentation and offerings to the gods. From this it was extrapolated that the use of white diamonds were offered to the Indian god Indra, the earthly incarnation of thunder and lightning, and black gems were offered to Yama, the god of the dead.

References to diamonds are found in many of our ancient texts and classical arts. The word “adamas” meaning unconquerable is found in classic Greek literature, yet this word is not used to refer to the diamond. It is thought that Pliny the Elder used the word in reference to the hard mineral that is diamond in the first century. The diamond is also seemingly referred to, astonishingly as it may seem, in the Old Testament, whereby the Hebrew word “Jahalom” (halom meaning to hammer or hit something with force) was used to refer to a stone with unrelenting hardness. Of course, as with all ancient texts we must be circumspect. We know with some confidence that the word “jahalom” is definitely translated as diamond. This fact originates from the 17th Century. It is debateable whether the same meaning was attributed to the original Old Testament texts.

Even in today’s market, it is the rarity and great beauty of a stone that will give it its inherent worth. The octahedron above shows how a stone can reflect its timeless magic, and because diamonds were held up as stones of great mysticism and magic they were even more coveted. As we have mentioned, diamonds have been enfolded in legend and mythology for many centuries, but as Christianity became more popular and spread across the globe, the legends and gods that people worshipped were dismissed as superstitions, and because of their place in mysticism, the value of diamonds diminished.

It was because of the introduction of new and advanced working procedures in the 16th Century that the diamond would once again be widely coveted. Because Indian mines were the only source of diamonds until the 18th Century, they quickly became drained of their bounty, but diamond deposits found in Brazil meant that supplies of rough diamonds were as abundant as ever. Many of them found their way to Antwerp, recognisably the diamond capital of the world.

In 1866, South Africa’s discovery of diamonds changed everything. De Beers, now universally recognised as a mining cartel began to operate in the area and is responsible for the prospecting and trading of diamonds on an unprecedented scale. Most of these rough diamonds were channelled towards Antwerp, which is still known as the world’s largest diamond centre. After the Second World War, the popularity of diamonds increased, resulting in a trading boom which still resonates today.

What is a diamond? ~ Mineralogy and Crystal Structure

Diamonds are formed in lattices of carbon tetrahedrons. The term ‘diamond’ is a mineral; crystallised and composed of carbon atoms.

It is not surprising to accept that diamond is the hardest substance known when you observe the way each tetrahedron is structured. In the centre of each tetrahedron is a carbon atom, the vertexes consisting of additional carbon atoms. This forms a structure that is three-dimensional. Within this structure, atoms link together where the attractive force between atoms is created by the sharing of electrons. In the world of chemistry these bonds are known as the strongest. To break a bond such as this would require an unprecedented force of energy. A diamond’s physical characteristics are determined by this internal structure.

To illustrate diamond’s strength, the diagram above shows the internal structure of a diamond, and also the internal structure of graphite, another form of crystallised carbon. Graphite has a crystalline structure in the form of hexagons which is two-dimensional, unlike a diamond octahedron structure which is three-dimensional. The two minerals are very different because graphite is constructed in layers of carbon, connected by weaker electrical forces. If you press too hard on a pencil it will break very easily. It does not have the strengthened three-dimensional internal structure of a diamond.

The Barringer Crater in Arizona in the USA is famous for a rare rough diamond called Lonsdaleite. The crater was formed by a meteorite hitting the earth. The rough diamonds were originally graphite, but because of the intense heat and high pressure during the fall of the meteorite, graphite was converted to diamond even thought the hexagonal structure of the graphite was kept intact.

Crystalline Shapes

The octahedron is not the only crystalline shape within diamonds. Nature ensures that diamonds are made up of a varied number of shapes. The octahedron is certainly number one. The cube is the next most common, along with rhombic dodecahedron. Rarer than these are trisoctahedron; hexoctahedron, or tetrahedron. Some diamonds are made up of twin crystals, or some with no visible crystalline shape.

Some larger diamonds; a prime example would be the Cullinan; the largest diamond ever found at 3.106 carats, belongs to this category.

The crystals within diamonds form gradually from the core, layer upon layer. The result is a multi-layered crystal, rather like the structure of an onion. In some diamonds, this ‘graining’ is clearly seen when the diamond is polished or split. When the most advantageous growing conditions are present the octahedron shape of diamond crystal will form. Small steps in the face of the crystal are evidence of resorption (when previously formed crystal dissolves. Resorption will also affect the sharpness of facets.

Trigons, triangular shapes, are on occasion found on the surface of rough diamond crystals. They are peculiar to octahedral faces; a reflection of internal growth development. An experienced diamond cutter will ascertain the growth pattern of a diamond from these lines.

Twins

‘Twins’ are ‘polycrystals’, compound forms that make up part of the rough diamond. Polycrystals can be further subdivided into ‘penetration twins’ and ‘contact twins’. Penetration twins are twins that have connected or ‘grown into’ each other while sharing a twin axis. External growth lines or knot lines can be visible through a microscope or loupe when such a stone is cut because of the differences in hardness of the crystals. Contact twins each have an octahedral surface. An environment rich in carbon is instrumental in the development of two crystal germs. They are known as ‘macles’ (see photograph above and below) and often have a flat appearance.

Properties of a Diamond

Diamond is an exceptional substance. We know that its coveted appearance is used for decorative purposes and investment – a thing of great beauty and luminescence. But the strength of this mineral is often called upon in manufacturing and engineering industries; used in medical equipment and high-tech manufacturing such as computers.

  • Diamond burns when accompanied by oxygen at a temperature of 800C and forms the gas carbon dioxide (CO2)
  • Diamond is converted to graphite, in absence of oxygen, from 1500C
  • Diamond is the substance with the highest thermal conductibility (600 – 1500 Watt/m Kelvin)
  • Diamond is the substance with the lowest thermal coefficient of expansion (0.0000001/C
  • Diamond repels water. It readily accepts grease
  • Diamond is not damaged by acids. It is however damaged by melted salts such as potassium nitrate (KN03)

Chemical Properties

Covalence (sharing electrons) creates strong chemical bonds between carbon atoms within a diamond crystal. The exceptional strength of a diamond, making it impervious to blows or outside influences is the result of the structure of chemical bonds.

Physical Properties ~ Hardness

Friedrich Mohs – Measures of hardness of a material will take into consideration various properties within that material. The scale of Mohs, a widely used method for measuring hardness, and developed by Friedrich Mohs in 1822 is commonly used for measuring this physical attribute. His scale ranged from 1 (softest) to 10 (hardest). He tested ten common minerals and placed them on the scale according to their ‘scratch resistance’. Any mineral that scratched another would receive a high ranking including scratching itself. In this way he arrived at his recognised scale.

Diamond has a hardness of ten, the hardest of all minerals, and only susceptible to scratches from another diamond. We have discussed that a diamond’s hardness is caused by the bond between carbon atoms. The depth of that hardness also depends on the direction of the atoms. A high density of atoms will result in a greater hardness, so there will be differences in hardness, meaning they can be cut using diamond powder in certain areas. The mineral’s extreme hardness does not mean that a diamond does not present fragility. A diamond crystal can be shattered.

Density

The density or specific gravity of a stone is expressed by g/cm3 and is the ratio of the stone’s mass to its volume. A diamond is about 3.5 times heavier than water (for the same volume): 3.51 g/cm3. When aiming to distinguish an imitation stone against the real thing, this ratio is used to ascertain each stone’s density. It can also be ascertained by weighing the stone in water and in air. Liquids with dissimilar densities are also used. If the stone sinks then it has a higher density level than the liquid.

Refractive Index

Part of a diamond’s allure to us is the way the light plays on its surfaces; its seductive brilliance and the colours that emanate from its facets. The refractive index measures the optical density of a substance; the higher on the index, the more efficient the substance bends light entering it. For a diamond this would be 2.42.

Conductive Properties

Of all substances known, diamond has the highest conductivity grade. Metals such as copper and silver have less thermal conductivity than diamond. It is the perfect insulator when at room temperature. Blue diamonds, however, evacuate heat quickly, displaying semi-conductor properties, essential in the computer industry.

Effects of water and grease on a diamond

Diamonds repel water which will slide from the surface where the stone has not absorbed it because of the very density of the carbon atoms in the crystal lattice of the diamond, yet a diamond will attract grease, which can be an advantage in manufacturing.

Resistance to acids

It has already been mentioned that melted potassium nitrate (KNO3) will damage a diamond, yet its unique properties protect it from acids. When a stone is being cleaned, it will often be immersed in boiling sulphuric acid, which dissolves dirt and impurities from the surface without damaging the precious stone.

Optical properties

It is this property of a diamond that seduces us into wanting to own one (or more) of these precious gems. Diamonds sparkle and scintillate, refracting light as it enters the stone, creating a show of colour and brilliance. These qualities will determine the value of the diamond, using the 4Cs as a guide, and gemmologists will closely follow these guidelines and evaluate a diamond according to Cut, Colour, Carat and Clarity, and any inclusions that may be present.

What is light?

Daylight is formed from different wavelengths from the visible spectrum which can be seen in a rainbow, or perhaps when light shines through a glass prism. It is how ‘white light’ is dispersed at different colours on the spectrum.

TV and radio waves, radar waves, X-rays, infrared rays, cosmic and gamma rays all belong to the unit of electromagnetic rays. The visible portion of the electromagnetic spectrum is light and transmits at a very high velocity. Its wavelength will determine the colour that is seen.

Behaviour of light in a diamond

An image is seen when light falls onto an opaque or smooth surface. We call this a reflection. When light falls onto a substance or object that is transparent, a part of the ray of light will certainly reflect, yet part of the ray of light will penetrate the substance and be diverted in different directions. This is the result of the light being refracted.

Reflection ~ in optical physics, all angles are measured from ‘normal’ (N). This is an imaginary plane, lying perpendicularly to the reflecting surface and intersecting the point where the light ray touches the surface. With the appearance of the phenomenon of reflection the angle of reflection (r) is equal to the angle of incidence (i).

Refraction and refractive index ~ when light enters a transparent substance (optically transparent) a section of the light is reflected, but still another part of it goes through the substance and is then refracted.

i>r
When a light ray propagates from an optically thin (air) to an optically dense medium (diamond), then it is broken towards the normal (N)

i>r
When a light ray propagates from an optically dense (diamond) to an optically thin medium (air, then the opposite occurs, the light ray is deflected from the normal (N)

A formula can be used to determine the refractive index (n) of a substance by using the formula below:-

The greater the refractive index of any substance (each substance contains at least one refractive index (n)) the more deeply the light is refracted when entering a substance or object.

The refractive index (n) is also the degree of light deceleration in certain substances:

  • Air has a refractive index almost equal 1, which means that the speed of light in air is approximately the same as its speed in a vacuum, i.e. 300,000 km per second.
  • Water has a refractive index of 1.33, which means that the speed of light in water is 1.33 times slower than its speed in a vacuum (or air). This corresponds with a speed of approximately 226,000 km per second.
  • Diamond has a reflective index 2.42, which means that the speed of light in a diamond is 2.42 times slower than in a vacuum (or air), approximately 124,000 km per second.

Critical angle (= total reflective angle)

It’s the brilliance of a stone caused by light penetrating the diamond and refracting out of it that is one of the unique properties that attract us to a diamond. When a ray of light leaves a diamond, it leaves from an optically dense medium (the diamond) and enters an optically thin medium (the air). The beam is actually diverted from what is considered as normal, yet when light penetrates the stone the reverse happens. For a certain angle of incidence (i) the refraction angle (r) becomes 90 degrees, i.e. the light beam can’t go further through the surface and reflects in the diamond itself. This angle of incidence is called the critical angle (g) or the total reflective angle. When crossing over from diamond to air the critical angle is 24.4 degrees. This means a light beam that is moving through a diamond and falls on a facet is then reflected in the stone when the angle to the normal (N) is greater than 24.4 degrees.

Coefficient of reflection

The coefficient of reflection is the ratio of the amount of light that reflects from a surface to the total amount of incoming light.

The coefficient of reflection of a substance is influenced by the angle of incidence (i), the refractive index (n), and the wavelength (^) of the incoming light.

Reflectivity meters, used for the identification of diamond are based on this principle.

Effects brought about by optical properties

Brilliance ~ a high refractive index is the light leaving a diamond, or the brilliance that emanates from the stone. It is well known that the most efficient shape of diamond to ensure this is the round brilliant cut diamond, although other polished shapes are almost as efficient.

Dispersion ~ the word dispersion relates to the fire emanating from the diamond. Fire and light are often used as descriptions for that intense sparkle and warmth we see when we observe a quality diamond. This effect is caused by the difference in the refractive index (n) for differing wavelengths. Refraction occurs when light leaves the stone and is not as strong for some wavelengths as it is for others. The rainbow effect we observe in diamonds is the result of white light entering the stone, then emanating in different colours. This is known as dispersion.

As you can see from the above table, the light ray with the longest wavelength results is red. This means that the light ray is the least refracted. In contrast, the highest refraction produces violet. In turn this means that the light ray has the highest refractive index. Dispersion is shown by increasing or decreasing wavelengths resulting in varying colours.

Dispersion can be seen most efficiently when the light waves transit from an optically dense substance to an optically thin. i.e. diamond to air, and will result in maximum dispersion when the angle of incidence (i) is just a bit smaller than the limiting angle (24.4 degrees). The dispersion coefficient of diamond is 0.044.

The shortening effect ~ the appearance of a diamond's characteristics closer to the surface than they are is called ‘the shortening effect’. The way the lens of the eye operates and light refraction from air to diamond results in this phenomenon.

I – actual depth I` - perceived depth
From the above diagram, a mathematical connection can be deduced between the perceived optical depth (PI`) and the actual depth (PI), so when a diamond grading is undertaken, this phenomenon will be applied to the grading. The inclusion will be 2.42 times deeper within the stone. This equation is an important one, as the resulting stone will have a weight grading which would be affected by any error in the cutting of the stone. Actual depth = 2.42 x apparent depth will be used by gemmologists to ensure that the stone is cut accurately to maximise weight and appearance.

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