Diamonds - their properties, origins and history

17 Apr 2007

By Robin

Diamonds have long captivated the human race, for their beauty and hardness, and for their natural fire due to the unique optical refraction and dispersion properties. It is thought that most people in the western world will own at least one cut diamond in their lifetime. Natural Diamond crystals are more of a rarity to be owned, for several reasons – they are highly valued as a cut stone, therefore it is difficult to obtain high quality natural crystals outside of the gem trade, and they are often very small in size.

Approx 20% of all mined diamonds are cut into gems, and the majority of the rest is used in Industry.

Chemistry

Diamonds are a native element and are composed of nearly pure crystallised Carbon and form under high temperature and pressure conditions. Graphite is also composed of Carbon and is the form that Carbon takes under low pressure and temperature conditions.

Diamonds are commonly white or colourless but can also form in a range of colours depending on the minute inclusions of other elements. Yellow is the most common colour, and brown is common at all localities, whilst rare colours such as blue and pink are highly prized in cut gems. Minute inclusions of Nitrogen will give yellow hues; Boron will create blue; pinks and reds are caused by defects in the atomic structure; and green may be caused through radiation damage of the crystal structure – as a rare natural phenomenon, or created in the laboratory. Black diamonds also occur, and are caused by mineral inclusions or irradiation. Some diamonds may also fluoresce under UV light.

In natural uncut Diamonds the lustre ranges from greasy to adamantine, and the translucent crystals may often take on a pearlescent appearance. The crystal faces may be smooth, finely pitted, finely striated or they may include triangular etch pits, also know as ‘trigons’ when the triangles are in the same orientation as the face they are on. 

Crystal faces are also commonly convex, giving a rounded appearance to dodecahedrons. Skeletal growth may also occur, with negative faces and bevelled edges.

Hexoctahedral crystal form with curved crystal faces giving a spherical appearance to the crystal.	Modified tetrahedron, with curved faces.

Natural Diamond crystal exhibiting complex skeletal crystal growth, and showing 'negative' faces.

Diamonds have a very high refractive index which gives cut stones their fire, and have relatively high density and high thermal conductivity. Diamonds are the hardest natural material known to man, and are assigned the number 10 on Moh’s scale of hardness. A diamond however is brittle due to the very regular crystal structure of the atoms, and will cleave perfectly in four directions. This means that whilst a diamond cannot be scratched (except by another diamond) if you drop a diamond it may fragment into pieces. This property is used by gem cutters.

The crystal form of Diamonds is isometric and the characteric forms are octahedrons, cubes and dodecahedrons.

Octahedral crystal form	Dodecahedral crystal form	Hexoctahedral crystal form

The crystals can often be more complex with combinations of faces from these shapes, or there maybe single crystals or clusters of intergrown crystals. Another characteristic shape is the spinel-law twin or ‘macle’ crystals which have a flattened triangular shape. Diamonds may also form as spheres, with rough surfaces named ‘Bort’, which is fact created by a large number of small crystals arranged in a radial aggregate.

A complex crystal form combining cube, octahedron and dodecahedron crystal faces. Fine striations are present on the dodecahedral faces, and fine pits on the cube faces.	A natural crystal exhibiting the complex form with cube, octahedron and dodecahedron crystal faces.

Natural diamond 'Macle'	Spinel-law twin, or 'Macle'

Occurrence and Distribution of Diamonds

Diamonds have been found across the world on almost every continent, and are not a rare mineral.

The world’s first diamonds are thought to have come from India, the Golconda fields worked from as early as 800 BC, and two famous diamonds – the Koh-i-Noor (‘Mountain of Light’), now a cut stone in the Queen Mother’s Crown (England), and the blue ‘Hope’ Diamond. India was the main source of diamonds until the discovery of diamonds in Brazil in the early 17^th Century. Diamonds were next discovered in South Africa in the 1860s. Other prominent locations today are Zaire (now Congo Republic), Angola, Botswana, Russia and Australia. Canada also has several diamond locations which are still to reach full production. Small quantities of diamonds have been found in the USA.

The famed Star of Africa

The world’s largest gem-quality diamond is the Cullinan Diamond found in 1905 at the Premier Mine, Transvaal, and weighed 3025.75 carats (approx 1.5 pounds). The crystal morphology suggests that this diamond is in fact only a cleaved section of a larger octahedron which has never been found. The crystal was cut into several stones, the largest is 530.20 carats known as the Star of Africa, which is set in the British Royal Scepter and now resides in the Tower of London.

The origin of Diamonds

Diamonds must form very deep under the Earth’s surface in order to have a high pressure and temperature environment. They are thought to form at depths of 140-200 km with temperatures above 950 degrees C. Diamonds then require a particular type of volcanic activity to bring them to the surface; partly because the magma must come from such a depth (most volcanoes have magma from 60km depth or shallower); and partly because they must be quickly accelerated to the surface - so that they do not become unstable with the dropping of temperature and pressure and convert to Graphite; nor are oxidised to carbon dioxide.

Diamonds were initially found in alluvial deposits – the sands and gravels that are transported by water in rivers, and deposited in the river beds – or carried to the ocean. These were eroded from their original locations and redeposited. Because Diamonds are so hard, and resistant to weathering they can be carried great distances from their sources before being redeposited. The diamond finds in India, Brazil and South Africa were all alluvial deposit finds, and it was not until 1871 that the source of Diamonds were eventually discovered as volcanic rocks, and were begun to be mined. Diamonds have also since been found in Glacial Tills.

Diamonds are found at the surface in two types of rare volcanic rock - Kimberlite or Lamproite.

Kimberlite is an potassic ultramafic rock rich in volatiles (or dissolved gases).  It is generally formed of larger irregular fragments in a finer groundmass. Weather Kimberlite is often termed ‘yellow ground’ and the unaltered Kimberite ‘blue ground’ due to their colour. Kimberlite forms very deep in the Earth and is found near the surface in volcanic pipes (named a diatreme), sills or dykes.

Kimberlite pipes typically have a carrot shape and are split into 3 zones – the ‘root zone’ which generally is connected by a feeder system which is believed to reach the upper mantle, the ‘main body’ where the pipe widens and is a mix of Kimberlite and brecciated country rock, and the ‘crater zone’ which originally consisted of a shallow crater and a surrounded Tuff ring, both of which was now often weathered away.

The emplacement of a Kimberlite has never been observed, but is thought to occur as a very fast (geologically-speaking) explosive event, with the dissolved volatiles rapidly coming out of solution as the pressure drops and propelling the Kimberlite further. The Kimberlite is thought to be emplaced as a ‘cold’ solid not a magma due the lack contact metamorphism and other thermal effects on the pipe walls, with multiple pulses shattering the already solidified material overhead.

The fact the emplacement of the Kimberlite is fast is the reason that we have diamonds, as the crystals do not have time to re-equalise and convert or pseudomoph to Graphite.

Lamproites are less gas-rich than Kimberlites and hence have a less explosive eruption. Lamproites similarly form very deep, and occur in pipes at the surface, but have wider pipe shaped more like a champagne flute. Lamproites have a different mineral composition to Kimberlites, but the diamonds do not seem to differ majorly.

Kimberlite and Lamproite are found in continental cratons, or along the tectonic plate margins and can vary widely in age. The Earth’s surface is made up of tectonic plates. The plates beneath the oceans average around 30 kilometres thick and the plates under continents 70km. The oldest parts of the continents are know as cratons and are relatively stable with little geological change in the last 2.5Ga years. Here the plates reach their thickest and may be up 140 – 200 km thick.

So how do the diamonds form?

It had been found that the diamonds occur within the included fragments or ‘xenoliths’ and are therefore older than the Kimberlite. The xenoliths are of two rock types – Eclogite (approx half and half Garnet and Clinopyroxene) which formed from ocean floor lavas which carried to depths by descending (subducted) tectonic plates, and Peridotite (averaging 50% Olivine, 40% Pyroxene and 10% Garnet) a common rock type of mantle material. Diamonds can be assigned to either E type or P type categories based on studies of their inclusions, and assumptions can be made about the origin of the carbon from which the diamonds formed. P type diamonds from Peridotite most likely formed from primordial carbon incorporated in the mantle whilst E type diamonds may be from carbon from organisms and carbonate minerals from the Earth’s surface carried down with the subducting plates.

It is thought that E type diamonds form at greater depths and temperatures than P type which are believed to form 150-200km depth and age about 3 billions years (based on the stable Achaean cratons in which the Kimberlite pipes are found).

As the diatremes are found only in cratons we can deduce that diamonds must form at depths of 140-200 km, and also must age about 3 billions years, since diamonds are older than the kimberlite pipes, and the kimberlite pipes must have been emplaced then as cratons have not geologically changed in the last 2.5 GA years.

There are several other interesting places in which diamonds have been found. Diamonds have been found as nanometre crystals in primitive meteorites, from carbon formed by stars and included in material when the solar system was first formed. Meteorites have also been known to create diamonds from the high pressure impacts on the Earth’s surface with tiny diamonds formed within the crator.

The exploration for diamonds today is much easier with the knowledge and technics we have now, with Geophysics looking for anomalies in the Earth’s magnetic fields or electrical resistivity, and chemical analysis of soils and sediments looking for ‘indicator’ minerals (minerals that formed under high pressure and temperature conditions and were brought to the surface with the diamonds) that may indicate the presence of a nearby pipe.

References:
- Diagrams from Atlas der Krystallformen, Victor Goldschmidt, courtesy of Mindat.org
- Diamonds and Gold. The Mineralogical Record, Vol 35 No. 1 – Jan-Feb 2004;
- Diamonds, the world’s most dazzling exhibition. The Natural History Museum, London – 2005;
- Diamond Properties, Geology, Exploration, Mining, Use. Alluvial Exploration and Mining. Rafal Swiecki – Feb 2006.

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