<Ul> <Li> Diamond crystallizes in the cubic system but graphite crystallizes in the hexagonal system . </Li> <Li> Diamond is clear and transparent, but graphite is black and opaque . </Li> <Li> Diamond is the hardest mineral known (10 on the Mohs scale), but graphite is one of the softest (1--2 on Mohs scale). </Li> <Li> Diamond is the ultimate abrasive, but graphite is soft and is a very good lubricant . </Li> <Li> Diamond is an excellent electrical insulator, but graphene is an excellent conductor . </Li> <Li> Diamond is an excellent thermal conductor, but some forms of graphite are used for thermal insulation (for example heat shields and firebreaks). </Li> <Li> At standard temperature and pressure, graphite is the thermodynamically stable form . Thus diamonds do not exist forever . The conversion from diamond to graphite, however, has a very high activation energy and is therefore extremely slow . </Li> </Ul> <Li> Diamond crystallizes in the cubic system but graphite crystallizes in the hexagonal system . </Li> <Li> Diamond is clear and transparent, but graphite is black and opaque . </Li> <Li> Diamond is the hardest mineral known (10 on the Mohs scale), but graphite is one of the softest (1--2 on Mohs scale). </Li>

Which one of the following is an allotrope of carbon