In physics, hardness is a measure of how resistant solid matter is to various kinds of permanent shape change, when a force is applied. Hardness can be measured on the Mohs scale where diamond is ranked as the hardest natural material. Diamond is made from carbon atoms joined by extremely strong covalent bonds. There are harder synthesised materials though; for instance, aggregated diamond nanorods, or ADNRs, are made by subjecting carbon-60 molecules to immense pressures.
Although it’s easy to forget, air molecules all weigh something, and their combined weight pressing down is what causes this pressure