In order to make a sparkling stone from a rough diamond, processing is necessary. This must be done very carefully because if mistakes are made the stone can disintegrate into many small fragments or can even burn up. The most important steps of the processing are: cleaving or sawing, bruting and polishing. Because diamonds are composed of layers they can be split (cleaving). When faceting the diamond, one must take account of the ‘grain’, the direction in which the crystals have grown.
In Venice, at the beginning of the 14th century, diamonds were cut for the first time. In the 17th century, Amsterdam became the world centre for the processing of, and trade in, diamonds. Today the main polishing centers are: Antwerp, Mumbai, Surat, Guangzhou, Tel Aviv and New York.
Cleaving (splitting) diamonds demands great knowledge on the part of the cleaver.The position where the stone is to be cloven or sawn is first marked with ink. Afterwards, the cleaver fixes the diamond to a cleaver’s stick with the aid of cleaver’s cement and, with another diamond, makes a V-shaped groove at the spot where the stone is to be cloven. He puts the blunt cleaving knife in the groove and gives a short blow with the mallet there. Then the diamond splits in two. The technique of cleaving is no longer practised very much. It has been largely replaced by sawing, or, with larger diamonds, by cutting with the aid of a laser beam.
For sawing diamonds, a thin sawing disc (0.05 to 0.14 mm thick) made of phosphor bronze is used, the edge of which is coated with a mixture of diamond powder and oil. The stone is sawn at 4500 to 6500 revolutions per minute.
The still uncut shape can be rounded by grinding the diamond against another diamond (the ‘polisher’). The grindings are retrieved and used for industrial purposes.
Now the facts must be introduced that allow the diamond to sparkle. This used to be done by fixing the stone in a lead dop (cap), which was clamped into a wooden tang (tongs) with a copper handle. The precise placing of the diamond in the dop was the work of the ‘setter’. Nowadays, the polisher puts the stone into the polishing tang directly. The facets are polished on the stone with a horizontally-revolving polishing scaife (wheel) coated with oil and diamond powder. The stone sometimes loses up to 70% of its weight due to the polishing process.
For centuries in some cultures, cutting as large a stone as possible out of rough material was paramount; a precisely symmetrical shape was less important. This is called India or Ceylon cut. It is however, very important for diamonds that the facets be cut precisely symmetrical, because then the light can be collected and reflected from all directions. In this way, optimum sparkle and brilliance can be achieved. Due to modern polishing equipment there are hardly any limitations in cutting shapes