xkcd

At least that follows some mathematical logic. Mohs scale of hardness is pretty close to pT scale in that sense, but there’s no mathematics or logic involved. It’s just a list of standard materials that define specific points on the scale. When you compare the results with a more logical scale, it looks neatly non-linar at first glance, but the closer you look, the less sense it makes. It’s just a list of exceptions to whatever rule you may have had in mind.

Doesn’t mean it’s a useless scale. You can totally use it for qualitative assessment of hardness, but steer clear of it when numbers and decimals actually matter.