Using diamonds to polish gems

Portal Earth Sciences
Diamond (Ancient Greek: ἀδάμας or adamas, “unbeatable”) is an allotropic manifestation of carbon that is found as a mineral but can also be produced in laboratories. In diamonds, the carbon-carbon bonds have a quadrilateral structure, which means that the atoms are bound in three dimensions; this partly explains the hardness to which the mineral owes its name. On the other hand, graphite, the most common carbon form on earth, has a flat crystal structure which makes it much softer and forms flaking layers. Diamond is the hardest material known to occur in nature and is therefore the benchmark for hardness 10 on the Mohs hardness scale. Only a few industrially manufactured materials, also made of pure carbon, are harder.

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In 2500 B.C. the Chinese are said to have used diamonds to polish gems. In India diamonds would have been worn as jewellery by that time and the Bible also mentions diamonds. Diamonds became known in Europe in the 6th to 5th centuries BC. An old Greek bronze statue with unprocessed diamonds originates from this period, which is nowadays in the British Museum in London. Diamonds have already been described by Manlius (16 A.D.) and Pliny the Elder (23-79 A.D.) in his Naturalis historia.

Modern diamond cutting with a cast iron turntable, diamond powder and olive oil was invented in 1456 by the Bruggeling Louis of Birch who dragged the Florentine for Charles the Bold. This was the beginning of the diamond industry in Bruges. After the silting up of the Zwin at the end of the 15th century, not only the port activity, but also the diamond industry shifted to Antwerp. After the fall of Antwerp, many wealthy Antwerp citizens fled from the Inquisition to Amsterdam, which marked the beginning of the diamond industry there.

In the 17th century the French explorer Jean-Baptiste Tavernier brought large diamonds from India and described diamond mining there.

Until the 18th century, diamonds were only mined in India. This is where the most famous diamonds in history come from. In 1714 diamonds were discovered in Brazil. Erasmus Jacobs found the first diamond in South Africa near the Orange River in 1866: the ‘Eureka’. Shortly afterwards, diamonds were also found in Kimberley. In 1888 Cecil Rhodes founded the company De Beers to control the diamond trade.

In the 20th century, important diamond deposits were discovered in Siberia, Canada and Australia, and it became possible to manufacture diamonds synthetically.

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There are many legends associated with diamonds; often magical or protective powers are attributed to them. Diamonds were the symbol of wealth and they are part of almost all crown jewels, treasure chambers and museum collections. The diamond is one of the “nine gems” in the Thai Order of the Nine Gemstones.

The Cullinan is the largest uncut diamond found on earth so far: 3106 carats (621.2 grams). The Cullinan was cleaved and cut and the largest piece, the Cullinan 1 (530.20 carats) was the largest cut diamond for about a century after it was cut. The largest cut diamond since 1988, however, is the Golden Jubilee (545.67 carats), which was cut by Gabriel (Gabi) Tolkowsky on behalf of De Beers and has been in the possession of the Thai king Bhumibol since 1997, who received it on the occasion of his 50th coronation jubilee.

Many diamonds for industrial purposes are also made synthetically. Synthetic diamonds can only be distinguished from natural diamonds in a laboratory. Researchers from the Carnegie Institution of Washington discovered in 2004 a process to synthesize within 24 hours diamonds that are more than 50% harder than natural diamonds.

Physical properties

The crystal structure
Diamond is the hardest material in nature. There are only two (industrially manufactured) materials that are harder, namely aggregated carbon nanorods and ultra-hard fullerene. Like diamonds, these are made up of carbon atoms. Diamond itself is a transparent crystal with a very high refractive index (2.417) and a high dispersion (0.044). In jewellery, the (sun) light is therefore brilliantly broken and reflected depending on the shape of the cut. In addition, the polished polished surface of the diamond stone does not become matt due to its high hardness.

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Because of its extreme hardness, diamonds are used in industry, for example for grinding, drilling, cutting and polishing and wire drawing. A diamond owes its hardness to its tetrahedron structure and is therefore harder when it contains fewer inclusions or crystal lattice defects. Diamond is relatively brittle due to its hardness. In vacuum, diamonds are transformed into graphite at a temperature of 1700°C and into air at a temperature of 700°C.