Diamond use to be the hardest known natural substance now it is the mineral lonsdaleite at 58% stronger then diamond is the first the second hardest is wurtzite boron nitride at 18% stronger then...
The scientific scale of hardness ranges from 1 to 10 on the "Moh's Scale" with talc at 1 and diamond at 10. So if by "hardness" you mean mineral hardness then the answer is diamond.