Diamonds are cut and shaped with special techniques that have been developed over the years. There are four basic techniques, cleaving, sawing, bruting and polishing. Before anything is done however the stone is studied carefully, sometimes, as in the case of a large stone, for weeks or months to decide the best way of cutting it to retain the best qualities of the stone and reduce the amount cut from the stone.
Cleaving is probably the most common and this is used to remove impurities and irregularities in a diamond. A rough diamond is placed in quick-drying cement and a sharp groove is then carved into the diamond with another diamond or perhaps a laser. This is done along previously established planes of weakness. A steel blade is placed in the groove and a sharp blow to the blade will split the stone at that groove. The diamonds, as there are now two, are removed from the cement.
Sawing is done when a diamond needs to be cut against a cleavage plane. This cannot be done with cleaving. For this a special phosphor-bronze blade is used rotating at over 15,000 rpm. However, these days, it is more common to use a laser to cut through a diamond.
Bruting is a method of creating a rough finish of the girdle of the diamond. With this method, two diamonds are positioned in a lathe and rubbed together.
Lastly there is polishing. This is what gives the diamond it's finish. This is done by using diamond powder on a wheel rubbing and smoothing the diamond as it turns. The polisher would inspect the diamond every few minutes and turn it to ensure each facet is polished to the same brilliance.