Lets put on our interesting physical deformities, grab our signature pets, head to the moon and hold the world ransom with our space rocks.
The Chelyabinsk object was estimated to be 11,000 tons. This didn't impact, it exploded in mid-air, and rattled some windows. But it should make a sufficient warning shot that will make the leaders of the United States, Britain, and the USSR sit up and take notice (and give into our ransom demands for cargo containers of diamonds to be dropped into the ocean so they can be collected and brought to our under-water base.) So as our opener, we need to send 11,000 tons of material to earth. Given the gravity on the moon is 1/6th the earth's , this would be like moving around 2,000 TON projectile on earth.
To give you an idea of the scale, that's like shifting around 3 Christ The Redeemer statues, or about 10 blue whales or your mom. And this is all just to rattle some windows and put on one hell of a light show, mind you.
We'll assume the moon rock is just an unguided projectile. I'm not even going to address the computational horsepower you'd need to put your moon rock onto a target even as small as a city, and the exacting level of calibration you'd need to be able to hit a moving target from a moving target over 300,000km. Because it doesn't matter.
Everyone is welcome to check or correct my math as its been a long time since I've had to do space physics, but accelerating 11,000 of mass to the 2.3Km/s for lunar escape velocity would require 26,394 Terrajoules, or 7,332 megawatts. That's over double capacity of the largest nuclear plant in the US. You could use a ~5-mega ton blast to propel your space rock as well - so about 4 nuclear missles worth of warheads - provided your space rock survives the blast intact and your launch area doesn't absorb any of the energy.
In short, people will notice. And unless you up that energy requirement even more, they're going to have a few days to do something about you and your little moon rock. (Like dispatch a suave MI-6 agent to infiltrate your moon base, seduce your chief scientist, and plant evidence that will turn you and your partners against each other. Or just use their own nukes to knock it off course/fragment it. Whatever feels right.)
Again, this is for a window rattler, not something that would actually impact on the surface.
Conclusion:
To weaponize space rocks, you're going to need nuclear energy. If you've got nukes, why wouldn't you just use those?
Yes, Mr. Bond. Why indeed? Ha. Ha ha. Muwahahahaha.
