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Ancient Chinese Technology:
Chinese Used Diamonds to Polish Sapphire-Rich Stone in 2500 BC
CAMBRIDGE, Mass. -- Researchers have uncovered strong evidence that the ancient Chinese used diamonds to grind and polish ceremonial stone burial axes as long as 6,000 years ago - and incredibly, did so 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 diamond worldwide thousands of years earlier than the gem is known to have been used elsewhere.
The 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.
Lu's work may eventually yield new insights into the origins of ancient China's trademark Neolithic artifacts, vast quantities of finely polished jade objects.
"What's most amazing about these mottled brown and grey stones is that they have been polished to a mirror-like luster," Lu says. "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."
Lu's subsequent X-ray diffraction, electron microprobe analysis, and scanning electron microscopy of the four axes' compositions 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.
To further test whether diamond might have been used to polish the axes, Lu subjected samples of the fourth axe, 4,500 years old and from the Liangzhu culture, to modern machine polishing with diamond, alumina, and a quartz-based silica abrasive. 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 next known use of diamond occurred around 500 BC; it was used after 250 BC in ancient India to drill beads. The earliest authors to reference what is likely diamond, Manilius and Pliny the Elder, lived in Rome during the first century AD.
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