What is hardest metal on earth?

The hardest naturally occurring metal on Earth is osmium. It has a Vickers hardness of about 4,700 to 5,000 kilograms per square millimeter (kg/mm²), making it one of the densest and most rigid elements. Osmium is a member of the platinum group metals and is found in nature mainly as an alloy with other metals. Due to its exceptional hardness and other desirable properties, osmium is used in various applications such as electrical contacts, fountain pen nibs, and in the aerospace industry for certain high-stress components.

Keep in mind that when we talk about the "hardest" metal, it often depends on the specific criteria used to define hardness (e.g., Vickers hardness, Mohs hardness, etc.), and the hardest material overall may be different from the hardest naturally occurring metal. For example, synthetic materials like diamond or cubic boron nitride are even harder than osmium, but they are not metals.