Maybe a rock’s atoms are closer together or something?
In: Physics
They’re not necessarily closer together, they’re just locked-in tight because they form crystals.
Metals and minerals (among other things) form crystals which are made of atoms which are essentially locked into position. “Soft” materials like rubber are a random mish-mash of *molecules* which can move around each other. In crystals, there are no molecules but rather huge arrays of locked-in atoms that span the entire length of, say, a metal bar. Since the atoms can’t move much, they can’t move out of the way when you press against them, and it feels hard.
With a piece of rubber, the individual molecules can literally slide past each other and stretch / move when you press against it, so it feels soft.
Then you have hard stuff like wood which is not a crystal but rather made up of a very rigid framework of lignocellulose with microscopic “soft” bits in between, but that’s different.
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