What @kori.dacosta said is probably closer to what you need. It does not need to be destructible at all (but it can, of course). It does not even require that advanced of a setup. In UE4 static meshes have material slots - each happy to display a different material.
Not sure how this is achieved in other 3d modelling software, but in 3ds Max you can assign IDs to each poly (and optionally name them). FBX exporters preserve those.
When you hit the ground (with a trace) you can sample its material and apply it to the relevant top ID of the extracted rock. There’s a bunch of nodes that help with that:
I once posted (years ago) a step-by-step for this here on AH but can’t find it. 