Diamond, one of the hardest known materials, could have a rival. A new form of carbon that is just as hard as the gemstone but as light as pencil graphite could theoretically be used for applications including gas storage and optoelectronics.

Carbon exists in different natural forms, called allotropes, depending on how its atoms bond with each other. Among the most well-known carbon allotropes are diamond and graphite, the substance used for pencils, although many more types can be made …