While diamonds are often thought to be the hardest substance on Earth, they are neither the strongest material overall, nor are they the strongest naturally occurring material. Carbon atoms may form a solid, ultra-hard structure known as a diamond if the conditions are just perfect. But the average density of carbon is only 1.5 times that of gold, so any object made entirely out of diamond would be very fragile. The strongest material on Earth by far is pure iron, which can be hard enough for useful applications. However, iron is a metal, not a rock. It can't be found in its natural state; it must be extracted from iron ore using chemical processes. Diamonds, on the other hand, are the hardest natural substance known to man. They can only be formed in the presence of oxygen and heat, both of which are rare properties in the universe. The next-hardest materials known to man are corundum, which includes sapphire and ruby, and these too are rarely found in their natural state. However, there are other substances that are harder than diamond. Calcite, which is what rocks such as limestone are composed of, can be hard enough for practical purposes. And some meteorites contain objects called black widows that are even harder than calcite. But none of these substances are found in nature anywhere on Earth, or even outside our planet.