Can somebody who knows about rocks weigh in here? I was under the assumption that pretty much the whole Earth crust was made of rocks, and that we’d run out of virtually any resource before we run out of rocks.

Basalt is extremely common, its not something that anyone will run out of.

We may run out of basalt sources that are close enough to farmland it can be spread on, but it would take a while. And trains would change the economics.

I'd imagine the types of rocks would matter. There are also a lot of ores that should remain in the ground, and not pulverized and spread, contaminating water and soil.

All the oceanic crust is made of basalt, so approximately 2/3 of the planet is covered by several miles of it. Unfortunately mining oceanic crust is impractical, for obvious reasons. However some oceanic crust got trapped between mountain ranges during continental collision, forming ophiolites. These can be suitable mining sites, together with LIPs (large igneous provinces) and the rocks of some shoshonitic (K-rich) basalt volcanoes.

Rock is not a scarce resource. The planet is literally made of it.

We're all just pond scum on a ball of molten iron.

Yeah, but it isn't an infinite resource. Eventually, I meant, inevitably, I wrote.

Nothing is infinite. For the purposes of grinding basalt rock to sprinkle on farms, it is essentially sufficiently abundant that it doesn't matter.