While diamond lost the title of "hardest material" to man-made nanomaterials years ago, a rare naturally occurring mineral called lonsdaleite may surpass both. Like diamond, lonsdaleite is comprised of carbon atoms but arranged differently.
Lonsdaleite is extremely rare and is naturally formed when meteorites containing graphite slam into the Earth. Through simulations it was demonstrated that lonsdaleite may be 58% harder than diamond, making it harder than any measured substance.
The simulation also tested wurtzite boron nitride - another rare material formed during volcanic eruptions - which may be 18% stronger than diamond. Until single crystals of both materials are grown or isolated, their hardness cannot be confirmed.