Ancient Chinese craftsmen 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.
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 scientific journal Archaeometry, places this earliest known use of diamond worldwide thousands of years earlier than the gem is known to have been used elsewhere.
Scientists had put the earliest use of diamond around 500 BC.
The latest work also represents the only known prehistoric use of sapphire.
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 colors.
Most other known prehistoric artifacts were fashioned from rocks and minerals no harder than quartz.
"The physics of polishing is poorly understood. It's really more an art than a science," said the first author Peter J. Lu. "Still, 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 centimeters, 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 Jiangsu Province; the fourth, discovered at a Liangzhu culture site in Zhejiang Province.
"What's most amazing about these mottled brown and gray stones is that they have been polished to a mirror-like luster," Lu said.
"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," he added.
Lu's subsequent X-ray diffraction, electron microprobe analysis and scanning electron microscopy of the four axes' composition gave more evidence that quartz could not have polished the stones: fully 40 percent corundum, the second-hardest material on Earth, the only material that could plausibly have been used to finish them so finely was diamond.
The use of diamond by Liangzhu craftsmen is geologically plausible, as diamond sources exist within 300 kilometers of where the burial axes studied by Lu were found.