The hardest known metal alloy, and the hardest known metal in general, is a type of carbon steel, Alloy 1090. With a tensile strength of .84 GPa (122,000 psi) and a yield strength of .64 GPa (67,000 psi), carbon steel is surpassed in hardness only by very hard nonmetals, such as ruby, diamond, or aggregated diamond nanorods.

The best high-carbon steels rate an 8 on the Mohs scale of hardness, relative to ruby at 9 and diamond at 10. The hardest metals are still somewhat soft in comparison to the strongest nonmetals, based on the strongest bonds in chemistry, the sp₂ bond between carbon atoms.

High-strength carbon steel goes through a process of tempering and annealing to make it harder. After this process is complete such steel is called strengthened steel, the hardest metal available. Carbon contents as great as 2% may be present in the hardest metal.

Another one of the hardest metals is tungsten carbide, used in both the military and for top-shelf sports equipment. Depleted uranium, the metal of choice for tank shells, is not harder than tungsten carbide or carbon steel but it is more dense, which is preferable for projectiles. The strongest single element is osmium, used for high-grade pencils because it can withstand the force of writing, even with a tiny tip. Osmium is $400 per troy ounce, much more expensive than the alloy-based metals.

Most carbon steels melt at 2800 °F (1537 °C), and osmium at 5491 °F (3033 °C). Iridium and titanium are also sometimes incorrectly called the hardest metals. Titanium has been used to build the world's deepest-diving submarines, however.

Sometimes very hard metals, like carbon steel, are undesirable because of their other properties. Carbon steel, for instance, is notoriously corrosive. Just because it is the hardest metal does not necessarily mean that it is the most useful for every job.