Carbon

Crystal structure of carbon allotropes
sp3 abc* diamond (A4) cF8 wide-gap semiconductor, metastable**
sp3 a hexagonal diamond (lonsdaleite) hP4 wurtzite form of diamond
sp2 ab graphite (hexagonal, A9) hP4 semimetal
  ab buckled hexagonal graphite hP4 deformation of graphite
  abc rhombohedral graphite hR2 deformation of smooth graphite sheets towards diamond
sp2 bct-4 tI8 stacked trans-polyacetylene with CC bonds instead of CH
sp2 H-6 cP6 stacked trans-polyacetylene with CC bonds instead of CH
sp2 K4 cI8 unstable
sp2 C-20 cF80  

* Stacking of virtual hexagonal layers
** See the phase diagram on Wikipedia

References