In 1226, diamonds were a king’s best friend. When Louis IX took the French throne at the age of 11, in 1214, he considered them so precious that he passed a law banning anyone but the monarch from owning them. Some rulers believed that carrying diamonds endowed them with special powers. The 6th-century Indian text on gems, the Ratnapariksa of Buddha Bhatta, declared: “The king who carries a beautiful diamond with glittering flashes has a force that triumphs over all other powers and becomes master of all neighbouring lands.” Today, it is a healthy bank balance rather than blue blood that ensures ownership.

Part of a diamond’s allure comes from it being the world’s hardest and purest substance. It is the only gemstone formed from one element - carbon - and is at least 140-times harder than any other mineral. It can be cut only using another diamond.

The word diamond comes from the Greek adamas meaning “unconquerable” or “invincible”. This was used to refer to the strongest materials - from which the weapons of gods were believed to be forged - and eventually became synonymous with diamond. Its hardness is due to its crystal structure. A neutral carbon atom has six protons and six electrons surrounding its nucleus, with four of the electrons able to bond with other atoms, resulting in a flat sheet of connected carbon atoms. In the case of diamond, all four electrons form bonds creating a strong tetrahedral unit.

These are not always arranged in the same pattern, so “rough”, uncut diamonds come in different shapes. The most common is an octahedron that looks like two four-sided pyramids stuck together at their bases.

Subterranean cauldrons

While most diamonds are transparent, some contain impurities that colour them. These are called “fancy” diamonds. Traces of graphite create black diamonds, hydrogen produces grey ones and boron makes blue ones. Green diamonds are thought to arise when radiation affects the stone during formation.

Scientists believe that most diamonds were created when prehistoric seabeds were forced down into the Earth’s mantle at ancient “subduction zones” - areas where great slabs of the crust press together, pushing one down beneath the other. In subterranean cauldrons - below 150 kilometres (95 miles) in depth - temperatures of more than 1,000C and pressures in excess of 50 kilobars provided optimum conditions for diamonds to crystallise from hot carbon-rich fluids. However, because diamonds are “metastable”, they can exist only deep in the mantle or at the surface. At the moderate temperatures and pressures in between they revert to graphite. The fact that diamonds are found at the surface today means they must have travelled upwards in explosive fountains of molten rock.

Evidence for such eruptions is found at the surface in kimberlite pipes.

These are the solidified remains of the vents through which diamond-loaded magma rocketed to the surface. The surface area of diamond-bearing kimberlite pipes can be anything from two to 150 hectares. Named after Kimberley in South Africa, where they were discovered, they have since been found in Australia, North America and Siberia.

Although most diamonds derive from kimberlite pipes, some have been eroded and carried far from their source by rivers and glaciers. It was in such “placer deposits” that diamonds were first discovered, in India.

The first mention of a diamond comes from a Sanskrit manuscript called the Arthasastra (The Lesson of Profit), dating from between 320 and 296bc. Even back then, good quality octahedral diamonds were coveted items, as the text demonstrates: ”(A diamond that is) big, heavy, capable of bearing blows, with symmetrical points capable of scratching (glass) vessel I revolving like a spindle and brilliantly shining is excellent. That (diamond) with points lost, without edges and defective on one side is bad.”

Traders exported diamonds from India. Initially, they were kept uncut as talismans, but later became incorporated into decorative items. A Hungarian queen’s crown, set with uncut diamonds and dating from about 1074, is considered one of the earliest examples of their use in jewellery. By the 1300s, the English and French royalty also wore diamonds. Possibly the first royal diamond wedding ring was given in 1477 by the Habsburg Emperor Maximilian I to Mary, Duchess of Burgundy.

Early on, the settings of jewellery tended to overshadow the stones, but as techniques for polishing and cutting evolved, and new sources were discovered in South America, diamonds began to feature more prominently in jewellery. Before the 1850s, they were still rare and associated only with the aristocracy, but an event in South Africa was to change that. In 1859, a teenage boy found an unusual pebble on the bank of the Orange River. It turned out to be a large diamond. The diamond rush that followed transformed the South African economy.

The centre of the boom was Kimberley, and a pit there became known as Big Hole. At 215 metres deep and 1.6km round, it remains the largest hand-dug excavation in the world. By the time it closed in 1914 it had yielded 2,722 kilograms of diamonds and helped make the De Beers company the world’s largest diamond business.

Mighty collision

Today, diamonds are mined in more than 25 countries. The world’s largest mine is the Argyle field in Western Australia - 10-times bigger than the Kimberley field and yielding 43 million carats a year. The four main diamond trading centres are Antwerp, New York, Tel Aviv and Mumbai (formerly Bombay), with Antwerp handling 85 per cent of all rough diamonds.

Only 20 per cent of diamonds are of sufficient quality to be made into jewellery. The rest are used in industry in cutting and grinding machines. It is thanks to such technology that we can now have spectacles made in one hour. In the future, diamonds may be used as high-frequency semi-conductors in space communications.

Geologists believe that diamonds form in most major impacts. This has led them to widen their search beyond kimberlite pipes and alluvial deposits.

In the past 20 years, they have found “micro-diamonds” in Kazakhstan in an area that was once subjected to a mighty collision between continental plates. They’ve also encountered traces of diamond in meteorite craters. In 1987, microscopic “nano-diamonds” were found in meteorites older than the solar system. Studies suggest they formed more than five billion years ago as radiation flashes from dying red-giant stars shot into clouds of methane-rich gas.

Understanding how these gemstones form has helped scientists to create synthetic diamonds in the laboratory. Using a method called chemical vapour deposition, researchers at the Carnegie Institution in Washington DC have made diamonds that are 50 per cent harder than natural ones. The technique involves bombarding hydrogen gas and methane with plasma in a chamber containing a small “seed” of existing diamond. The resulting chemical reaction prompts carbon atoms to fall on to the seed and arrange themselves in the same crystalline structure as the original diamond.

A company called Gemesis began selling synthetic stones in 2003 and last year produced between 500 and 600 carats of yellow and orange diamonds.

Natural diamonds of this colour fetch pound;20,000 to pound;27,000 a carat, but Gemesis’s equivalent price is pound;2,300. If their products catch on, the gems once reserved for royalty could become a mineral of the masses.

* Some of the world’s most spectacular diamonds will go on show at the Natural History Museum in London next month. They include Ocean Dream, the world’s largest natural fancy deep blue-green diamond, and the Pumpkin, a bright orange one worn in a ring by Halle Berry when she collected her Oscar in 2002. Historic items on display include Mogul pieces demonstrating past cutting techniques and a third-century Roman ring. The diamond exhibition runs from July 8 to February 26.

Tel: 020 7942 5000

www.nhm.ac.uk

GEM-LOVERS: (above) King Louis IX of France (1214-70) made the ownership of diamonds strictly reserved for royalty; (below) a miner holds the uncut Cullinan diamond shortly after it was found in South Africa, in 1905

BOOKS

* Diamond: The History of a Cold-blooded Love Affair. By Matthew Hart. Fourth Estate pound;7.99. History, geology and adventure combine in this tale of Canada’s diamond rush.

* Rocks and Minerals. By Chris Pellant. Dorling Kindersley pound;12.99. Learn to identify common rocks and minerals.

* Blood Diamonds: Tracing the Deadly Path of the World’s Most Precious Stones. By Greg Campbell. The story of Sierra Leone’s blood diamonds.

QUALITY CONTROL

Jewel experts assess the quality of diamond gems using the “four Cs”: cut, colour, clarity and carat. Different types and shapes of cut include the princess, emerald, round brilliant and Asscher. Colour is graded on a scale from D to Z, where D is transparent and diamonds graded above K have an increasingly yellowish tint. Clarity measures the flaws in a diamond.

Surface flaws are called blemishes; those inside are known as inclusions.

Carat is the measure of weight. A five-carat diamond weighs one gram.

Perfectly cut, large, flawless, transparent gems fetch sky-high prices.

FAMOUS DIAMONDS

Cullinans I and II

This 3,106-carat diamond was the largest gem diamond ever found. It was discovered in 1905 at the Premier Mines in South Africa and named after the mine’s owner, Sir Thomas Cullinan. The Transvaal government bought the stone and gave it to King Edward VII for his 66th birthday in 1907. When cut in Amsterdam, it yielded nine major gems, including the Cullinans I and II. Cullinan I, also called the Star of Africa, is a pear-shaped stone of 530.2 carats. It is set in the head of the King’s Royal Sceptre (pictured left) in the British Crown Jewels, which reside in the Tower of London. The Cullinan II, weighing 317.4 carats, is set in the British Imperial State Crown. When the Cullinan was discovered it had a smooth cleavage on one side, suggesting it may have been part of a much larger crystal. However, no “missing half” has been authenticated.

Koh-I-Noor (Mountain of Light)

It was once said that whoever owned this 186-carat diamond (below, centre front) ruled the world. Its provenance dates back to 1304 when it was owned by India’s Rajah of Malwa. After passing into the possession of the Mogul emperors, it is said to have been set as one of the peacock’s eyes in the Peacock Throne of Shah Jehan, who reigned in the early 1650s.

When the state of Punjab was annexed to British India in 1849, the East India Company gained the diamond. The company presented it to Queen Victoria in 1850 as part of its 250th anniversary celebrations.

The stone was displayed at the Crystal Palace Exposition, but when visitors complained that it lacked fire, Victoria had it re-cut. It featured in the coronation crown of the late Queen Mother.

Millennium Star

The pear-shaped 203-carat Millennium Star (far left) was the centrepiece of a collection put on display at London’s Millennium Dome in 2000.

Discovered in the Congo, and without a single flaw, it has been described as the most beautiful diamond in the world. The pound;200 million collection, comprising the Star and 11 blue diamonds, was the target of a foiled robbery. Thieves used a JCB digger to break into the Dome, but the police had received a tip-off and were waiting to arrest them.

Hope

This 45.52-carat diamond (left) has a reputation for bringing bad luck to those who come into contact with it. Owned by the French monarchy, until it was stolen during the revolution in 1792, the blue Indian stone is thought to have been bought by King George IV of England in the 1800s and then sold in 1830. It was purchased by Henry Philip Hope, from whom it takes its name. One of his heirs, Lord Francis Hope, sold it in 1901 to a New York diamond merchant before dying in poverty.

A French broker who later bought the stone committed suicide; an eastern European prince gave the diamond to an actress who was shot the first time she wore it; and a Greek jeweller who sold it to a sultan was thrown over a cliff. The diamond is now a star attraction at the Smithsonian Institution in Washington.