Diamonds are the hardest known naturally occurring substance.
Talc is the softest mineral found on earth, reaching just 1 Mohs scale of hardness ,
It is often used to make talcum powder.
The Mohs scale of mineral hardness is based on the ability of one natural sample of mineral to scratch another mineral visibly. Talc is scored ‘1’ and Diamonds score ’10’
Scientists have made something known as nanocrystalline Diamond (hyperdiamond, ultrahard fullerite) which is harder than Diamond. Also, elements like Cesium(Caesium, Cs) and Rubidium(Rb) are softer than Talc, but are highly unstable at room temperature and are not found naturally.