Harrow and Hillingdon Geological Society

Previous Meetings

Diamonds

Diamonds are comprised of the pure form of the mineral element carbon. It was deposited in Kimberlite pipes about 3 billion years ago. This is the hardest natural substance known to man and is rated 10 on the Moh scale of hardness. The carbon may be of several different shapes including, cubic, octahedral and dodecahedral and can be cleaved in four directions. Great pressure and temperature has produced a very dense element scaled at 3.51 compared to graphite at 2.2 and quartz at 2.66. Diamond has a remarkable ability to reflect and refract light. The vast majority are type 1a and contain nitrogen in the lattice, giving rise to a yellowish colour. 2a are big diamonds, very low in nitrogen and virtually colourless. 2b contain no nitrogen but have boron in the lattice. A carat is 0.2gm and the largest diamond ever found was 3106 carats.

London was the original trade centre for diamonds and depended historically on the Jews and many of the stones came from India. The Indian product was at its peak in the 16^th C. Stones were found later in Brazil, 1851-Western Australia, 1868-South Africa and in 1918 Zaire, which is now the second largest producer in the world. These are younger, very small and only 5% are gem quality, most are used in industry. Natural pink diamonds are found in the Argyle mine which is an open pit. In Canada there are enormous resources of diamonds, where the kimberlite has been eroded and become a lake. These are marketed under their own system. China has an increasing industry and there are also some in America.

The cutting of these stones requires diamond blades or powder, but today modern stones utilize computer programmes for design and are cut by lasers. Diamonds can be produced synthetically in the laboratory from graphite, a technique developed in Sweden in 1953. In 1970, General Electric used enormous pressure and heat to convert graphite to diamond or crystallised carbon. This is not synthetic as it is a natural process. Up to 1996, very few synthetic stones were being used in jewellery but now synthesis is used widely. At 1,400ºC and at 50,000 atmospheres pressure, 3 carats take about 100 hours but near perfect cubo-octrahedral stones are produced. In 2002, high pressure/temperature diamonds were produced in USA and these were called cultured. They also converted human remains to diamond. Yellow diamonds contain a lot of impurities and if treated with heat and pressure continue to evolve. This is natural and not synthetic and can improve the colour. Black diamonds have also become popular and these can be reverted to graphite. In 1950, chemical vapour deposition allowed diamonds to be grown on diamonds and in 1976 using a mixture of methane and hydrogen, vapour diamonds were grown on a non-diamond substrate of silicon.

This was a good comprehensive, well-illustrated talk with some lovely slides. David exudes enthusiasm and obviously has many years of experience.

	Diamonds can be found in a wide range of colours. These are a few examples.
	Some diamonds have black inclusions. These are known in the trade as 'Coal Mines'. Nowadays lasers are used to drill down to them, turning them white and lessening the impact on the eye.

 

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