Because stones or rocks are a good example of hard things having no compromise. They are hard and unforgiving. It’s an example of an absolute, whereas all other things can be changed in some way.
Stones do not retain or transmit heat very well. Chef’s roll-out dough on a pastry stone (usually marble) because it keeps cool. Go out on a hot day and pick up a stone. The side that has not been in the sun will be cool to the touch.