Everyone knows that diamonds are special. Besides their glamorous beauty and special meaning, they're also the hardest minerals on Earth.

A new study suggests that even people in ancient China appreciated the wonders of diamonds. Some 6,000 years ago, people in China may have used diamonds to polish their stone axes. That pushes back by several thousand years the date for the first known use of diamonds.

To reach this conclusion, Peter Lu, a physicist at Harvard University, studied four ceremonial burial axes that were found in two tombs in southern China. The axes dated back to between 4000 and 2500 B.C.

Lu used special microscopes and other high-tech equipment to figure out that the axes were mainly made up of three minerals. The most abundant mineral was a type of aluminum oxide known as corundum. Rubies and sapphires are colored forms of corundum.

Finding corundum was a surprise because it's the second hardest mineral on Earth. Because diamond is the only mineral that's harder than corundum, Lu proposed that the ancient Chinese must have used diamond to shape and polish their axes.

To test this theory, Lu used a diamond saw to slice one of the burial axes. Then, he tried polishing the new surface with three materials: diamond, corundum, and quartz. Only diamond produced a surface that was as smooth as the original.

The new research also suggests that people in Stone Age China could have used diamond to polish jade and shape it into various objects. These ancient people may have been much more technologically savvy than experts had ever suspected.—E. Sohn