Maybe a rock’s atoms are closer together or something?
In: Physics
Not all rock is hard. Soft rock was quite popular in the 1980s.
What makes hard rock and metal hard? Mainly the drums. The tempo tends to be fast, and the vocalist is often loud and raspy.
I believe the number of electrons around each atom is higher, which makes the substance more dense and/or harder.
It’s a mixture of type of material/materials it is made of and the exact mixture/composition of everything and time and pressure applied to it. Like graphite, cubic zirconia and diamonds all made up of carbon but their molecular structure varies due to time, pressure, etc.
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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