Ancient Chinese crafts-men might have learned to use diamonds to grind and polish ceremonial stone burial axes as long as 6,000 years ago, US researchers said on Wednesday (Feb. 16th).

Researchers at Harvard University have uncovered strong evidence that the ancient Chinese used diamonds with a level of skill difficult to achieve even with modern polishing techniques.

The finding, reported in the February issue of the journal Archaeometry, places this earliest known use of diamonds worldwide thousands of years earlier than they are known to have been used elsewhere. Scientists had put the earliest use of diamonds around 500 BC.

The latest work also represents the only known prehistoric use of sapphires.

The stone worked into polished axes by China's Liangzhu and Sanxingcun cultures around 4000 to 2500 BC has as its most abundant element the mineral corundum, known as ruby in its red form and sapphire in all other colours.

Author Peter J. Lu said: "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."

Lu's work may eventually yield new insights into the origins of ancient China's Neolithic artifacts, vast quantities of finely polished jade objects.

Lu studied four ceremonial axes, ranging in size from 13 to 22 centimetres, found at the tombs of wealthy individuals. Three of these axes, dating to the Sanxingcun culture of 4000 to 3800 BC and the later Liangzhu culture, came from the Nanjing Museum in China; the fourth, discovered at a Liangzhu culture site in Zhejiang Province.

Using an atomic force microscope to examine the polished surfaces on a nanometer scale, he determined that the axe's original, exceptionally smooth surface most closely resembled although was still superior to modern polishing with diamond. The use of diamonds by Liangzhu craftsmen is geologically plausible, as diamond sources exist within 300 kilometres of where the burial axes studied by Lu were found. These ancient workers might have sorted diamonds from gravel using an age-old technique.

"I imagine that Neolithic craftsmen were constantly experimenting with new tools, materials and techniques," Lu said.

Editor: Catherine