Natural diamond is an allotrope, or form, of carbon crafted deep in the Earth. Other carbon allotropes include graphite, which is relatively soft, and fullerenes such as buckyballs and carbon nanotubes, which are exceptionally stable.

(See "Legendary Swords' Sharpness, Strength From Nanotubes, Study Says.")

The newly analyzed carbon material is a glass-like substance that factories have made for about 30 years for use in chemistry, electronics, and other purposes.

Until now, however, no one had studied what would happen to the material when placed under high pressure.

"Graphite is always soft, and diamonds are always hard," said Ho-Kwang "David" Mao, a high-pressure scientist at the Carnegie Institution for Science in Washington, D.C., and co-author of a new study on the glassy