Okay, someone has asked me for a ruling on Big Frickin' Rocks and High Places. Since the ruling I wrote at the very beginning was not very clear, I will endeavor to make it more clear here. I have also added a slight rules modification to falling damage, which I don't think anyone will find disagreeable.
Buying Rocks
Rocks cost 1 GP per 8 lbs of weight. I based this on costs for iron pots, which is a bit expensive for rock, but what the heck.
Falling Damage
A 200 lb rock does 1d6 damage per 10 feet it falls. For each halving of weight, it requires +10 feet to do the same damage. For example, a 100 lb rock does 1d6 damage per 20 feet it falls. Each doubling of weight adds +1d6 damage per 10 feet fallen. For example, a 400 lb rock does 2d6 damage per 10 feet it falls.
ALWAYS round weight down. A 199 lb rock is treated the same as a 100 lb rock.
Damage is maxed out after 200 feet, regardless of weight. For extremely un-aerodynamic objects such as trees in full bloom or hollow spheres, damage is maxed out after 100 feet, regardless of weight.
As an example, a 3,200 lb rock, dropped 200 feet, would do 100d6 damage. That is more than enough to ruin almost anyone's day.
Soft objects (flesh, rubber) does 1/4th damage, and allows a Fortitude save for 1/2 damage.
Targetting
Setting up the rock to fall and then dropping it is a standard action. If the fall is less than 100 feet, the attack is resolved during your action. A rock which is classed as Large-size or smaller requires a to hit roll against the target's AC. If they don't know you are there, you are considered to be flanking, with the usual penalties to their AC. A rock which is classed as Huge-size or larger targets a SQUARE, which as AC 4.
The range increment for dropping a rock is 50 feet, and this will affect your to hit roll.
If a rock is dropped more than 100 feet, the to hit roll is handled on your initiative on the NEXT ROUND. If the target has completely left the square you were targetting, you automatically miss. If a new target has stepped into the square, you get to roll against them, instead.
Spots & Saves
You get a spot check to notice a rock falling from more than 30 feet overhead. You get this spot check on the same round that the to hit roll is made. By weight, the DCs for this spot check are:
8 lbs - DC 12 (tiny)
60 lbs - DC 11 (small)
450 lbs - DC 10 (medium-sized by volume)
3,600 lbs - DC 9 (large)
28,800 lbs - DC 8 (huge, area effect 5 ft x 5 ft)
230,000 lbs - DC 6 (gargantuan, area effect 10 ft x 10 ft*)
2 million lbs - DC 2 (collossal, area effect 20 ft x 20 ft*)
Round all weights DOWN for purposes of size. A 25,000 lb rock is "large". If you fail this ridiculously easy spot check, you will take damage as appropriate for what was dropped on you.
*Note that gargantuan rocks divide their weight by 4, and collossal rocks divide their weight by 16, for purposes of damage done to any individual square.
If you succeed on the spot check, you are allowed a Reflex save. This is at DC is 15 for huge-size rocks or smaller, DC 20 for gargantuan rocks, DC 25 for collossal rocks. A successful Reflex save negates all damage, and moves you a number of squares in a random direction until you are out from under the rock.
Invisible Rocks?
Dust of disappearance will be blown off the stone by the wind of falling. Normal invisibility will break upon attack (which happens as soon as you drop it) and improved invisibility can not be cast on objects.
I will apply similarly harsh logic to all similar attempts to get around the spot check.