But the search for ultra-hard crystals continues to turn up promising candidates. Last year, Yongjun Tian of Yanshan University in Qinhuangdao, China, and colleagues studied a cubic form of boron nitride – rather than the hexagonal form Chen studied – which they claimed was also harder than diamond.
The experimental details of that study have been criticised, but Chen believes Tian’s material still bears investigating. “The main challenge is to understand the atomistic mechanism underlying this extraordinary result,” says Chen.