A diamond measures a 10 on the Mohs scale of hardness, making it the hardest natural substance known to occur on Earth. This raises the question of how the stones are cut if they are so hard. There are a number of techniques used in diamond cutting, including basic cleaving and the use of a grinding wheel called a scaif. These techniques have been developed over centuries of jeweling, and most cutters use techniques that were originally developed hundreds of years ago, because they are still the best way to handle the gems. Diamond cutters usually combine several processes to create a sparkling, faceted gem from a rough stone, in a painstaking job where the stone may lose as much as 60% of its weight.
The most ancient cutting process is cleaving. To cleave a diamond, the cutter places a chisel at a point of weakness in the stone and taps it with a mallet, causing the gem to split. If the weakness was misjudged, this can destroy the stone. If the diamond cutter judged correctly, the stone will be split into workable pieces which can be individually refined. Medieval gem cutters cut their cleaved stones with other diamonds, lubricating the surfaces with oil and grinding away at the stone to reveal facets. Some cutters still use other gems as part of their cutting and polishing process.
In the 15th century, the scaif was developed. A scaif is a polishing wheel that is kept liberally lubricated with oil and diamond dust. A cutter can use a scaif to polish a gem held in a dop, a padded holder that protects the stone while it is being worked on, only revealing the side that is currently being polished. The scaif changed the face of diamond cutting, allowing cutters to create symmetrical and even facets that bring out the true sparkle and shine of the gem. By playing with angles, cutters created unique and beautiful gems for setting in jewelry.
In the 20th century, another tool was added to the diamond cutting arsenal: a diamond saw. These saws are steel blades that are lubricated with an oil and diamond dust mixture that is continually reapplied as the stone is worked. Because the saws can generate a great deal of heat, the stones may be cooled as they are worked on in special cooling holders. This is also the case with a scaif, and is one of the reasons that cutting the gems is such a painstaking process, because the cutter must take a break every time the stone starts to warm up. If the stone contains any water in small bubbles or hairline cracks while it is being cut, the heat can cause the water to boil, and the gem may crack or explode.