Diamonds and pencil lead are both made of the exact same element: carbon.

Diamonds and graphite are known as allotropes of carbon, meaning they are different physical forms of the same chemical element. In graphite, each carbon atom is bonded to three others in a hexagonal pattern, creating flat sheets held together by weak Van der Waals forces. This structure allows the layers to slide over one another easily, which is why graphite feels greasy and functions as an effective lubricant and writing material.In contrast, diamonds form under extreme pressure and heat, typically found 150 to 200 kilometers below the Earth's surface. In a diamond, each carbon atom forms four strong covalent bonds with its neighbors in a tetrahedral lattice. This three-dimensional arrangement is incredibly stable and dense, resulting in a Mohs hardness rating of 10, the highest possible for a natural mineral.The discovery of this shared chemistry dates back to 1772, when French chemist Antoine Lavoisier performed an experiment to burn a diamond using a giant magnifying glass. He proved that the only combustion product was carbon dioxide, confirming that diamonds consist of carbon. Later, in 1797, English chemist Smithson Tennant demonstrated that burning equal amounts of diamond and graphite produced the same amount of gas, further solidifying their identical chemical nature despite their wildly different physical properties.