Precise cut for sparkling jewels

Not until they are given the right cut do precious stones reveal their true value. And they only fetch the highest prices if the facets are even and exact. However, the grinding process – which has hitherto been performed exclusively by hand – leaves little remaining of the valuable uncut stone: 66 to 70 percent fall to the ground as dust, while only a good 30 percent eventually sparkle in the light as a precious jewel. But which of the numerous cuts will make the most of the raw gemstone in question? Experienced lapidaries have an instinct for it.

For the first time ever, a grinding machine is challenging this collected experience: On average, it uses 15 percent more of the volume of the uncut stone. The machine has been in use with Paul Wild gem-cutters near Idar-Oberstein for three months, and has already transformed over a hundred lumps of rough stone into sparkling gems. “The machine – a CNC grinding machine with 17 axes – first maps the surface of the uncut stone,” explains Dr. Karl-Heinz Küfer, head of department at the Fraunhofer Institute for Industrial Mathematics ITWM in Kaisers-lautern, who developed the software for controlling the machine with the help of his colleagues.

“To do this, narrow bands of light are projected fully automatically onto the uncut stone, and its geometry can be determined from their curvature. The computer takes ten minutes to determine the image of the enclosed gemstone awaiting grinding, and sends the appropriate commands to the process control unit. The 17 axes ensure that the milling head can move along any desired path and grind the facets to an accuracy within ten micrometers – the gemstones become perfectly geometrical.” For comparison, hand grinding achieves an accuracy of about 100 micrometers, or the width of a hair. Hand-polished gems appear less exact, their facets and polished edges seeming to be slightly rounded.

The fully automated system takes an average of 20 minutes to give an uncut stone its facets. The machine has to work with extreme care and therefore allows the precious dust to fall rather more slowly than a skilled lapidary who has an instinct for the correct grinding pressure. On no account must the precious stone be allowed to get too hot, as this could cause it to split.

During polishing, however, the machine works faster: Whereas the skilled worker repeatedly has to wipe the stone clean and carefully inspect it, the machine sets the polishing time automatically depending on the size of the facets and the type and weight of the gem. “With uncut gems of average quality, the system will pay off within a year or two,” Küfer estimates.