Th ere are two stable isotopes of carbon. If natural carbon consists of 98.938% ¹²C and the accepted atomic mass of carbon is 12.0107 u, what is the isotopic mass of a ¹³C atom? Diamond is made of pure carbon and has excellent mechanical, electrical, and light transmission properties. Recently, scientists made some diamonds out of ¹³C and found that they had even better physical properties than “normal” diamonds do.