Almost everyone knows that carbon can take on more than one form, with diamond and graphite being the most common, and with the fullerenes, carbon nanotubes and this year's Nobel prize-winning graphene close behind. Now, evidence is mounting for the existence of a transparent form of carbon – bct-carbon – with a body-centred tetragonal (bct) structure something between graphite and diamond, first studied in simulations by Koichiro Umemoto and colleagues of the University of Minnesota, Tokyo and Tsukuba.

In separate work, Xiang-Feng Zhou of Nankai University in China and colleagues have made further studies with simulations. While their work is still theoretical, it does explain the experimental observation of a transparent allotrope of carbon formed in the compression of graphite. Remarkably, the material is harder than diamond and actually dents the diamond anvils used to provide the pressure to produce it.