Allotropes of carbon are promising materials for the next generation of electronics. In particular, boron-doped synthetic diamond, which is typically highly disordered, provides a powerful stage for studying superconductivity in a three-dimensional granular system. The authors find that, even deep in the metallic regime, heavily doped granular diamond still shows pronounced quantum effects in its transport properties, which needs to be accounted for---and could be exploited---in diamond-based SQUIDs and nanodevices.