Researchers have discovered evidence indicating that Chinese craftsmen might have learned to use diamonds for the grinding and polishing of ceremonial stone burial axes, as far back as 6,000 years ago. Prior to this most recent discovery, scientists had put the earliest use of diamonds at around 500 BCE.

The findings were reported in the February 2005 issue of the journal Archaeometry. The research also represents the only known prehistoric use of corundum, the mineral group that encompasses ruby and sapphire.

One of the researchers, Peter J. Lu, a graduate student in physics at Harvard University's Graduate School of Arts and Sciences, studied four ceremonial axes, ranging in size from 13 to 22 centimeters, found at the tombs of wealthy individuals. Three of these axes, dating to the Sanxingcun culture of 4000 BCE to 3800 BCE and the later Liangzhu culture, came from the Nanjing Museum in China; the fourth, was discovered at a Liangzhu culture site in Zhejiang Province.

The stones which comprise forty percent corundum - second only to diamond in hardness - must in all probability have been finished with diamond. "It had been assumed that quartz was used to grind the stones, but it struck me as unlikely that such a fine finish could be the product of polishing with quartz sand," says Lu. Using an atomic force microscope to examine the polished surfaces at nanometer resolution, he determined that the axe's original, exceptionally smooth surface most closely resembled - although was still superior to - modern polishing with diamond. "What's most amazing about these mottled brown and grey stones is that they have been polished to a mirror-like luster. It's absolutely remarkable that with the best polishing technologies available today, we couldn't achieve a surface as flat and smooth as was produced 5,000 years ago."

The use of diamonds by Liangzhu craftsmen is geologically credible. Diamond deposits exist within 150 miles of where the burial axes studied by Lu were found. These ancient workers might have sorted diamonds from gravel using an age-old technique where wet diamond-bearing gravels are run over a greased surface such as a fatty animal hide; only the diamonds adhere to the grease.

Lu's co-authors are Paul M. Chaikin of New York University; Nan Yao of Princeton University; Jenny F. So of the Chinese University of Hong Kong; George E. Harlow of the American Museum of Natural History; and Lu Jianfang and Wang Genfu of the Nanjing Museum. The work was supported primarily by Harvard University's Asia Center, with additional support from MRSEC grants and Princeton University's Department of Physics.