Diamonds have long been coveted for their hardness. Diamond ranks at 10 out of 10 on the Mohs scale of mineral hardness. A diamond has an absolute hardness of 1500 as measured by a sclerometer. The next closest mineral is Corundum at 400, followed by Topaz at 200. Interestingly, though, diamond is no longer the hardest substance known, although it remains the hardest to occur naturally. Scientists have discovered another material that usually exists at a lower hardness rating than diamond, which can be transformed under extreme pressures to form a synthetic substance harder than a diamond. Lonsdaleite is often known as hexagonal diamond, since its crystal lattice structure is hexagonal, versus cubic as in diamond. Lonsdaleite usually has a Mohs hardness rating between 7 and 8, but under extreme pressure it can be made harder than diamond. This has been discovered in meteorite samples and replicated in a laboratory. However, Lonsdaleite only exists in space, so it is in very short supply.