The sheer speed of an object at orbital velocity (8 km/s) is like the kinetic energy of 7.5 kg of TNT for every kg of mass. At the speed of an object falling from near the Moon, it's like 14.5 kg of TNT for every kg of mass. Meteors normally enter the atmosphere at even higher speeds, and are famously destructive if they don't burn up in the atmosphere enough for their energy to be dissipated.
The idea of turning that into a weapon has been pondered, but at the moment is not practical and so plays little role in strategic thinking. The near-term idea was to use tungsten rods dropped from orbit. That would seem to be designed to penetrate hardened defenses, not to do general damage, so a bit specialized.
At some point operations in space will make it possible to take material being mined from an asteroid or on the Moon and do something like this. It's still pretty far off, but considering the potential destruction, it seems to me it would be good for people to be aware of what could be done and what would be involved to do it. Surely our analysis of the value of space exploration, and how best to do it, needs to include this.
Since you could aim objects to enter at a near-vertical angle you could minimize how much the atmosphere slows them down and burns them away. So maybe the shortest path involves determining how to select appropriate boulders and shape them enough to be missiles. Is this an appropriate short list of necessary qualities?
- a conical shape,
- Solid enough to not break up before being close to the ground, but fractured enough to air burst once it is
- Perhaps coated with an ablative material, also to help it get close to the ground
- As dense as possible, so a bit of an iron meteor maybe, or a chunk of ilmenite if you are doing it from the Moon. If you have managed to start mining platinum group metals, they would work wonderfully but you probably wouldn't want to waste them.
And then there is the question of how much stuff would be needed to do big damage, say city-destroying level. The kinetic energy of a ton of material in the vicinity of the Moon is equal to the Hiroshima bomb, but much of that would be lost during atmospheric entry. If the material was prepared as in the list above, about how much of it would result in an explosion like Hiroshima?
Notes: TNT equivalent considers a ton of TNT to be 4.2 x 109 J of energy, and speed can be considered as kinetic energy with the formula $E_k=0.5mv^2$. The Hiroshima bomb exploded at a height of about 600 m.