You may have read about the rarity of gold, partly because gold is a highly coveted metal. It's attractive, valuable and doesn't corrode or tarnish like many other metals. Gold jewelry that is well cared for can stay in its original state for thousands of years. Gold is indeed considered to be a rare metal – 5,000 tons of the Earth provides only about 20 grams of gold – but many other metals are much rarer.
TL;DR (Too Long; Didn't Read)
In terms of abundance in the Earth’s crust, the rarest metal is francium, as there's less than 1 ounce in the world at any given time. However, you could describe many man-made metals as being even rarer because they barely exist at all.
What Is Metal
Metals are all around you – the gold, silver or platinum jewelry you wear, the iron, steel and copper that make up your car, and the aluminum foil you use to cover your food. Most metals are malleable, deforming then returning to their original shape when small loads are applied to them, but remaining flawed under greater pressure. Almost one-quarter of the planet consists of metals, and metals make up about two-thirds of all elements. The strength, high melting point, and thermal and electrical conductivity of metals account for their wide range of applications.
Two of the world's rarest metals are rhodium, which is estimated to be three parts per billion in the universe, and osmium, which is estimated to be about 0.6 parts per billion in the universe. By comparison, aluminum and iron make up 8.1 percent and 5 percent of the Earth's crust, respectively. Other natural metals are much rarer than rhodium and osmium because they are unstable, meaning they don't have a single stable, naturally occurring isotope. For example, less than 1 ounce of francium exists in the world at any given time. This is because it doesn’t last long. Its half-life is 22 minutes. After 22 minutes, half of it decays to another element, such as astatine or radium.
A rare metal could also be described as one that only exists for a short period – it doesn't exist at all most of the time. The highly radioactive man-made metals flerovium, oganesson, moscovium and livermorium have half-lives of 2.1 seconds, 0.9 of a second, 0.09 of a second and 0.06 of a second, respectively. Perhaps the rarest man-made metal is tennessine because no half-life has ever been recorded. Only a few atoms of each of these metals have ever been made.