There are actually different ways of measuring strength or hardness. Diamond is the hardest material, but it can sometimes be fractured. Artificial diamonds can be stronger than natural diamonds due to super-pure crystal latices, and doping with other elements.
Diamond is the strongest…but it depends what you mean – you wouldn’t make a bullet proof vest from diamonds – bit expensive!!
Graphene is also really strong – its carbon in another form, in fact it may even be stronger than diamond. Graphene is a completely new material– not only the thinnest ever but also the strongest. As a conductor of electricity it performs as well as copper and as a conductor of heat it outperforms all other known materials. It is almost completely transparent, yet so dense that not even helium, the smallest gas atom, can pass through it. Andre Geim and Konstantin Novoselov got the Nobel Prize in Physics 2010 for its discovery!!!