The compressive strength of a perfect diamond cristal is in the range of 220–470 GPa, depending on the direction you compress. (X. Luo et al, J. Phys. Chem. C 2010, 114, 17851–17853; DOI: 10.1021/jp102037j)
To cite this article’s introduction:
Usually, diamond is used under nonhydrostatic conditions, such as a diamond indenter in the nanoindentation test and as diamond tips of the diamond anvil cell (DAC) in ultrahigh-pressure research. Therefore, theoretical investigations into the mechanical properties of diamond under nonhydrostatic conditions should be important.
In experimental work, the compressive strength of diamond can be roughly obtained from the strength of DAC.
And the conclusion:
From the mechanism under compressive deformation of diamond, we can estimate that the limit strength of DAC should be about 470 GPa.