http://www.bbc.co.uk/news/world-africa-34867929
The Karowe mine is about 500km (300 miles) north of the Botswana capital, Gaborone. Botswana, South Africa, is the world's largest producer of diamonds, and this has been an exceptional week for the Lucara Diamond firm, as they have also discovered an 813-carat and a 374-carat gem-quality white-diamond stone at the same mine in the same week.
There are lots & lots of diamonds produced in this planet but far fewer are at the surface. A diamond is composed of pure carbon in crystal form (the soot from a badly-burning fire is also made of carbon, as is the graphite in your pencil). Carbon is one of the 3 pillars of life (carbon, oxygen & hydrogen). Diamonds need such low-heat, huge pressure & time (a minimum of 1 billion years) to be formed that, essentially, the only place for them to be made is within cratons. Cratons are ancient, stable regions of rock that underlie continental crust, and also extend deep into the mantle.
Having been formed, the diamonds need to be brought to the surface if humans are to find them. The typical method for this is volcanoes, but it needs a very special volcano... Most natural diamonds are formed at depths of 140 to 190km (87 to 118 miles), whilst typical depths for volcanic magma is 11-15km - far above those diamonds. There are, however, deep mantle plumes of rock that flow from the bottom to the top of the mantle (there are currently 2, directly opposite each other: one under Africa & the other under the Pacific Ocean). Those plumes start at the core / mantle boundary, which is 2,900km (1,800 miles) below the surface.
One very curious fact about diamonds is that the carbon that is their source can come from either inorganic or organic sources (this information is sourced from isotopic analysis:- the 13C:12C ratios indicate the source). Inorganic carbon comes directly from the mantle, and itself originates from dead stars, but perhaps even more interesting is organic carbon as a source. Organic carbon is from life that was originally on the surface of the planet but, through the processes of plate-tectonic subduction, entered the mantle.
The previous paragraph is extraordinary! The ancestors of most modern life appeared during the Cambrian explosion of life, which was ½ billion years ago. Yet diamonds take a minimum 1 billion years to form, and that can only start after the carbon gets down there.
Itsy-bitsy little diamonds are expensive, even if flawed (they are used to make drill-heads, as they are the hardest things on earth), but gem-quality diamonds are even more expensive. Last April a flawless 100-carat diamond was sold for $22.1m (£14.8m) at Sotheby's in New York. What size of gems will this stone produce?
The largest diamond ever found was the Cullinan Diamond (also known as the Star of Africa, found in 1905), a 3106.75 carat (621.35 g, 1.37 lb) stone. Most of the gems cut from that stone ended up in the British Crown Jewels, including the 530.2-carat Cullinan I which is set into the Sceptre-with-Cross.
Cutting a gem-diamond is a vastly skilled job. First, it is difficult: how can you cut the hardest material in the world? Secondly, all diamond stones contain flaws & imperfections when first found, whilst the final gemstone is required to be perfect to demand the highest price. The skill of the gem-cutter, then, is to be able to obtain the largest & greatest number of perfect gems from the source stone. Here is a story (which is possibly not true) sourced originally from Matthew Hart in his book Diamond: A Journey to the Heart of an Obsession:
The 3,106 carats of the Cullinan diamond were given to Asscher Brothers of Amsterdam (the Jews of Amsterdam were fabled then as now for their skill in cutting diamonds, and Joseph Asscher was considered to be the greatest cleaver of his time).
Joseph placed the diamond within a vice within a secure room. He then spent the rest of the day simply observing it, very closely. It was the largest gem-diamond that he had ever seen. Good lord, it was the largest gem-diamond that anyone had ever seen.
Diamonds are the hardest materials on earth, but shatter easily (in common with all crystals). Joseph needed to build up a picture of the diamond's flaws, of the planes where it would split easiest. He needed to peel away the poor stone from the perfect stone; to release the gems that lay within the source.
He normally spent a couple of hours in observation before making the first strike. That first strike was the most important of all. It had to be exactly right, and you only got one chance. Joseph spent 6 days with the Cullinan. At the start of the 7th, he picked up the specially-designed knife and prepared to strike. This was not going to be a tap. It needed strength, as well as accuracy.
It is said that Joseph had a doctor and nurse standing by. He gave the Cullinan one sharp, heavy blow. The knife broke into pieces. The Cullinan was unchanged. With a steady voice, Joseph asked an assistant to bring him a second knife. Joseph gave the Cullinan a second strike and it split into two through a defective spot, which was shared in both halves of the diamond. Joseph promptly fainted & fell to the floor.
The Cullinan was split and cut into 9 major stones and 96 smaller stones.
--------- Alex Kemp