Diamond, which is one of the allotropes of carbon, is used for drilling. Diamond is the hardest substance known whose density is3.05 gm/cc. The valence electrons of C are strongly held in a C--C binds. Because of the strong covalent bond in between C--C atoms, diamond is the hardest substance known so far.

The substance having same elements in same physical state but different structural arrangement is called allotropes. And the property of an element to exhibit allotropes is called allotropy.
Examples of crystalline form of carbon are diamond, graphite, fullerene etc.