  Diamond is the hardest known natural material. Its hardness makes it useful in industry, and its high dispersion of light when cut makes it appear very beautiful to the eye. The name 'diamond' comes from the ancient Greek word adamas, meaning 'invincible'. Diamonds have been valued for their beauty for thousands of years, and have been used for cutting and engraving for just as long. A diamond is a transparent crystal of bonded carbon atoms bound in a tetrahedron. When first removed from the ground, most diamonds are roughly rounded crystals; some are colourless, while others are pale shades of yellow; red, orange, green, blue, brown, and even black. These raw uncut stones lack the brilliance of gems which have been cut to increase their internal reflection properties.  Almost half of all the natural diamonds mined in the world still come from central and southern Africa, although significant
sources have recently been discovered in Canada, India, Russia, Brazil, and
Australia.Diamonds are formed deep in the earth, at depths of at least 150 kilometres, where the high temperature and pressure compresses carbon molecules into a compact diamond crystal lattice. Diamonds are sometimes found inside long-dormant and solidified volcanic lava pipes, in an igneous mineral called kimberlite. Carbon found in diamonds has been shown to come from both inorganic and organic sources. Some diamonds were formed from inorganic carbon deep in the Earth's mantle; others were created when organic matter near the surface was pushed down deep into the Earth as its surface plates pushed into each other. Learn more about plate tectonics on our volcanos page. However they were formed, diamonds that eventually reach Earth's surface are usually very old, ranging in age from under 1 billion to over 3 billion years. Once diamonds have been brought to the Earth's surface by magma in a volcanic pipe, they can erode out of the rock and be distributed over a large area. Small diamonds can also form in other naturally occurring high-pressure, high-temperature situations. For example, the impact craters where meteors strike the Earth show evidence of diamond formation. In fact, these 'microdiamonds' are used as to identify ancient meteorite impact sites.  Diamond's density is 3.52 g/cm3. Carbon atoms in diamond form a crystal lattice, bonding tetrahedrally. The tetrahedral arrangement of the atoms is the source of many of diamond’s properties.Carbon atoms in graphite , on the other hand, connect differently, forming sheets of atoms one on top of the other; graphite is soft, dark in colour, and slippery when in powder form. Comparing the structure of these two substances, both composed of carbon atoms, we see that the difference in their properties is due simply to the way the atoms are bonded together. Some diamonds are actually harder than others. The hardest type, quite small octahedral diamonds, are found in New South Wales in Australia, and are used to polish other diamonds. It is thought that they are the hardest type because their crystal form was grown in one stage; other diamonds were formed in many stages, with additional growth of crystal adding to their size but also adding flaws and defects, thus affecting their hardness. The hardness of all diamonds compared to other natural materials makes them very useful in industry, especially for use in cutting and grinding tools. Diamond can be used to cut, polish or wear away any material, including other diamonds. They are used in drill bits, saw blades, abrasive papers and engraving tools. Other Properties of Diamond  ToughnessToughness is a material's ability to resist breaking when hit forcefully. While diamond is more fragile in some orientations (it can be split), its overall toughness is exceptionally high. Colour Diamonds can be nearly any colour, although yellow and brown are the most common. Coloured diamonds contain impurities (various atoms replacing some of the carbon atoms in the crystal), or irregularities in their structure. Pure diamonds are transparent and colourless. Combustibility Like other forms of carbon (coal, for example), diamonds will burn, at a temperature of 800° Celsius, providing that enough oxygen is available. Optical properties Diamond acts like a prism, breaking light that enters the crystal into different wavelengths to form a rainbow of colours. This effect, multiplied by the many faces of a cut stone, gives a finished diamond a sparkling appearance called fire. In addition, diamond's high index of refraction can easily cause total internal reflection of the light, increasing its brilliance. Thermal properties While diamond is an electric insulator (it doesn't easily pass electricity), it is a good conductor of heat; in fact, it's one of the best. This makes it useful in the manufacture of electronic semiconductors. Diamonds as Gems Diamonds that have been prepared for use in jewelry are rated according to clarity, the number of carats, the colour, and their cut. Clarity is a measure of how clear the diamond is. Clarity is affected by inclusions, which can be foreign materials in the crystal, or structural imperfections like tiny cracks, which can cause the diamond to appear cloudy. Only about 20% of all diamonds mined have a high enough clarity to be used as gems. Carat weight is a measure of the mass of a diamond. One carat is one fifth of a gram, or exactly 200 milligrams. Diamonds of less than one carat are measured in points, where one point is equivalent to a hundredth of a carat, or 2 mg. Colour measures how close to perfectly transparent a diamond is. But few gem-quality diamonds are perfectly colourless; most white diamonds appear transparent, but have some nitrogen atoms replacing carbon atoms; this impurity can cause a slight yellow tint. The hue and intensity of the colour will help determine the diamond's value. For example, white diamonds with a yellowish tint are usually less valuable, while those with a pink or blue colour can be extremely valuable. If a diamond has a very strong colour, it's rare, and its value is usually higher.  Cut describes how the diamond has been shaped and polished into a gem. There are well-described mathematical guidelines for the angles and face lengths to be cut in order that the final gem reflect the maximum amount of light. For example, the modern round 'brilliant' cut diamond has 57 facets (polished faces), 33 of which are on the crown (the top half), the other 24 on the pavilion (the lower half). The purpose of the crown is to diffuse light into various colours; the pavilion reflects light back through the top of the diamond.  A well-cut diamond can appear bigger than it actually is, and with better colour and clarity. Diamonds can also be cut into various shapes to accentuate the features above, and to make them fit into various types of jewelry settings. A large uncut diamond is usually cut into one good quality gem and several smaller lesser quality ones.Sometimes diamonds are given a poor cut in order to increase their weight, since many people place more value on the number of carats. However, when this is done the result is usually a bigger diamond with greatly reduced sparkle. The Cullinan Diamond, part of the British crown jewels, was the largest gem-quality rough diamond ever found (in 1905), at over 3100 carats. The largest gem-quality cut diamond is the Golden Jubilee, a 546 carat , yellow-brown diamond. The largest flawless and colorless diamond is the Centenary Diamond, which weighs 274 carats. The most expensive diamond (based on what was paid for it) was the a 100 carat, pear-shaped flawless diamond sold to a Saudi Arabian sheikh by Sotheby's for over $16 million. 
The Hope diamond is the largest dark blue diamond in the world, at 45.5 carats. It was found in India many centuries ago; at that time it was uncut and weighed 112 carats. Sold to King Louis XIV, it was first cut down to 67.5 carats and became known as the Tavernier Blue diamond.Later the diamond disappeared, but surfaced in again in 1830 at 45.5 carats, having been recut. It was purchased by Henry Thomas Hope, an English banker. After changing hands several more times, it eventually ended up on display in Washington, D.C., where it can be seen today. Synthetic diamonds are real diamonds that are created by a technological process. Recently the techniques for producing these have improved, so that gem-quality synthetics, as well as industrial grade diamonds, can be manufactured. Diamond experts can tell a manufactured diamond from the natural kind, but this may no longer be true in the future as processing techniques improve. Diamond simulants are diamond look-alikes that are made from some other material. The most familiar is the zircon. |