What is the hardest rock and what is its compressive strength (PSI)?
Diamond, since it is a natural mineral, would be considered a 'rock.' Diamond has a bulk modulus (reciprocal of compressibility) of 443 GigaPascals (GPa). One GigaPascal is equal to 14503.77 PSI. Therefore, the incompressibility of diamond is about 6.425 million PSI or 437,200 Atmospheres.
Diamond was once thought to be the hardest and the most incompressible material on earth, either natural or man-made. Although diamond is still considered the hardest, the bulk modulus of the metal osmium has recently been found to be 476 GPa. That is over 7% more incompressible than diamond, but it is never found pure enough in nature to challenge the diamond.
Scott Wilber, President, ComScire - Quantum World Corporation